The first thing to know about goblins is that they're terrible. The
only races as bad at fighting as goblins are humans and kobolds, and the
humans make up for it with superior organization and technology.
Goblins are also very slow, have low carrying capacity, and take
penalties to hit in daylight. Goblins do have a couple of things going
for them, though - they have 90' darkvision, they don't weigh as much
as a man or dwarf, they can apparently semi-domesticate wild beasts, and
they have some of the best shamans and witchdoctors of any beastman
race (only hobgoblins are comparable).
Despite these strengths, pure goblins remain a pretty terrible choice for a field army. Their tactical weaknesses are immense, and require significant strategic advantages. With their darkvision and cave-dwelling, Viet Cong-style strategies seem to make a lot of sense to me: live in a heavily-trapped and laboriously-extended cave complex where regular human units dare not go, make night raids from one of its many entrances or exits, and then get back in the cave by dawn. In this case beast-cavalry can be used to locate good raiding targets, extend your raiding range, or delay pursuing units chasing your raiders back to the lair. This seems to me about the best an isolated low-tech goblin village or warband can hope for. Most of these fights will be at platoon-scale.
Frankly a goblin domain shouldn't play all that differently - it may need to farm on the surface, but everything that can be underground probably should be. On offense they raid, and on defense they play guerilla from their tunnel system. They just don't have the strength and mobility (outside of massed beast-riders, which are rare and expensive) to engage in fair fights with humans.
So here are some tribal goblin units:
Redcaps: 1/2/3 Irregular Foot, AC 2, HD 1-1, UHP 6, ML 0, 4 polearm 10+ melee
Redcaps are veteran goblin warriors who delight in terror and slaughter. They wield mismatched, hooked polearms, favoring modified scythes, and wear stinking leather caps stained with blood. Mechanically, they're 1st-level goblin assassins. They often strike from ambush, and may deploy hidden in obscuring terrain at the beginning of a battle (if fighting at night, redcaps may hide in any terrain). Note their location and facing - the first time they activate, they are revealed and placed on the map. They are also revealed if an enemy unit attempts to enter their hex, or if the obscuring terrain is removed by magic or fire. When attacking a unit in the flank or rear, or a unit which is disordered, they gain an extra +2 to hit (for +4 total if flanked or disordered, or +6 total if both) and deal an extra point of damage if any of their attacks hit (just as lances do on a charge).
Best-case scenario for redcaps is deployed in the flanks during an ambush or envelopment scenario, where they can rear-charge units who have run up against the main goblin line (potentially at +8 to hit (+2 charge +2 flank +2 disordered +2 backstab) and +2 damage from polearm-charge+backstab, which has the potential to wipe most human units). Consequently, if you're playing humans, it behooves you to use light units to check obscuring terrain for redcaps, or at least to block their charges.
Redcaps demand 15gp/mo in wages, for a company TCO of 2.25kgp/mo. I estimate their BR at around 0.75, but this does not account for their ability to deploy hidden.
Redcap Riders deployed on wolves would be hilariously strong on the rear-charge, even if they couldn't hide. Maybe I should do stats for them. At first warg (dohoho), you could turn most goblin units into redcaps in much the same way you'd make humans veteran: one extra attack from assassin damage bonus, +1 morale, ability to deploy hidden if infantry, ability to backstab in melee, and +12gp/mo wages. Following the veteran guidelines, around 1 in 4 goblin mercenaries is a redcap.
Deathcap Bowmen: 1/2/3 Irregular Foot, AC 2, HD 1-1, UHP 6, ML -1 unpredictable, 2 shortsword 10+, 2 poison shortbow 10+
Deathcap bowmen are goblin archers who soak their arrows in a variety of poisons and toxic molds painstakingly grown in underground fungal gardens. They get a lot of exposure to other, less-deadly fungal compounds too, and their grip on reality is somewhat tenuous. They wear wide, brightly-colored hats with down-turned brims vaguely resembling the cap of a mushroom. When a deathcap unit rolls a natural 1 on a shortbow attack throw, in addition to gaining a depletion marker, they take 1 UHP of damage and must roll shock. Two other variants to consider are madcaps (poison causing confusion) and nightcaps (poison causing sleep).
By the rules in the book, deathcaps have a BR of around 8. 0.5 of that is the unit itself, and the remaining 7.5 of that is just the poison (based off of a Potion of Poison, which can be used to envenom projectiles). I'm a little skeptical that that's reasonable. Sure, poison attacks are pretty good, but the delivery system is terrible. This isn't a wyvern or a purple worm where you get stuck in and then start making poison attacks every round with good THAC0. These are goblin archers with a maximum range of 5 hexes (when firing at cavalry or ogres), which many units can close in a single charge without giving them a chance to fire (and they can't ready to fire on a closing unit, because they're irregular). They also can't withdraw or disengage. If you put them out front in the opening ranged fight, human ranged units can withdraw to avoid the poison attacks, and the deathcaps are susceptible to being tied down in inescapable melee, in which case they 1) lose the ability to fire, and 2) probably get crushed because their AC and morale are both bad. But, if you keep them behind the main line and wait for the enemy to close, they still can't fire into the melee and take a -4 volley overhead penalty to hit anything behind it. Basically, because they're irregular and it's a ranged attack, they're going to have a hard time using it well.
I suspect that good use of deathcaps relies heavily on terrain. Placing them in the edge of obscuring, elevated terrain with an open field of rough terrain in front of them (mud, a river ford, whatever) is their absolute best-case scenario, because the rough terrain slows down melee units trying to close with them, the obscuring terrain provides a bonus against ranged fire, and the elevated terrain gives them a bonus to hit and inflicts a further penalty on ranged fire against them. But that's a mighty specific set of circumstances.
Comparing them with other units in the BR 7.5-8.5 range is also instructive. Consider: orc boar-riders, elven horse-archers, human cataphracts, ogre heavy infantry. It's a tough field. Would you rather have a company of deathcaps than of any of these units in a typical battle? Probably not. Deathcaps don't have anywhere near the survivability of any of those units (lacking AC, HP, and withdraw capabilities), and even if they manage to hit the sort of high-HD foes they need to be hitting for their damage output to compare to ogres or boar-charges, that extra damage is still gated on a poison save, which high-HD monsters have a reasonable chance of making. I just don't see it.
I really think a more-correct BR is somewhere in the 3-4 range. I would probably pick deathcaps over orcish crossbowmen at BR 2 because at that point I could get a bunch of them, but I probably wouldn't choose them over hobgoblin longbowmen at BR 4 (who are actually Loose Foot with OK speed, longer range, and good HP). Ultimately the problem is that they're both fragile and unreliable, which is a hard sell.
Now, if you put them on flying mounts, which fixes a lot of their fragility problems, we might be talking...
Goblin Bat-Archers: 3/6/9 Flyer, AC 2, HD 1-1, UHP 6, ML +1, 2 shortbow 10+ ranged, 2 shortsword 10+ melee
I'm not sure exactly how much goblins weigh, but it isn't much. Giant bats are a perfect mount for cave-dwelling, night-raiding goblins, and are faster than wolves. This unit also benefits from the fact that all flying units are FLY movement type, even when they would normally be irregular - this means that they can withdraw away from missile fire. Equipped with leather armor, shortbows, and shortswords, bat-archers are a good choice for harassing human forces pursuing goblin raiders on foot. Tactically, they're good for engaging enemy archers in the opening phase, keeping light cavalry occupied, and pursuing retreating units. Flying+ranged is also strong in hilly terrain, because you have great lines of sight. They don't hit all that hard, though...
Wages are 12gp/mo per bat+goblin pair. Monthly supply cost is 64gp for the bat and 2gp for the goblin, and specialist cost is 90gp/mo for a company, so TCO is right around 4.75kgp/mo for a company. Compared to wolf-riders at a similar price point, bat-archers are more fragile and don't have the brutally-effective charge attack, but they're faster and will almost never come under melee attack. Frankly they're also pretty annoying to kill with ranged attacks, because in addition to being able to Withdraw, enemy archers also have to Volley Overhead to hit them. Their battle-rating is around 3.75, well below that of wolf-riders. I'm not really sure how to handle availability on any of these goblin cavalry variants, and the fact that these are basically flying horse-archers certainly doesn't help.
The natural extension of this is deathcap bat-archers, but since I'm not sure how to properly price poison, there's not much more to say about them.
Goblin Bat-Lancers: 3/6/9 Flyer, AC 2, HD 1-1, UHP 6, ML +1, 2 lance 10+ melee, 2 javelin 10+ ranged
Another light-cavalry variant. Bat-lancers are useful in the opening phase of the battle for disrupting shield-walls with javelins, and then after the melee is engaged they can easily get behind the lines and charge the enemy rear. Again, this unit benefits significantly from being a Flyer instead of Irregular Mounted, because it can Disengage to pull out of hairy melee fights and Withdraw to avoid missile damage. Wages and TCO should be pretty close to the same as bat-archers, availability should be pretty comparable to that of wolf-riders. Same skills required, and the occupational hazards remain significant: your mount won't kill you, but gravity might. Their battle-rating is around 2.75.
Goblin Shrew-pack: 1/2/3 Irregular Foot, AC 6, HD 1, UHP 8, ML +2 unpredictable, 4 bite 9+
While giant shrews may be too small to ride, they make excellent warbeasts due to their high AC, multiple attacks, and aggression. They also breed readily in goblin tunnels, and share the goblins' taste for delicious bugs. This is a pack of 100 giant shrews, armored in leather and driven by 20 goblin handlers in leather with spears, shields, and high-pitched whistles. Shrew wages are 6gp/mo, and their supply cost is 2gp/mo. The shrew-herders demand 75gp/mo in wages each, plus 3gp/mo for serving as infantry and 2gp/mo in supplies, and 130gp/mo for an armorer to maintain all that leather. TCO comes out to a mere 1.75kgp/mo, with a battle rating around 2 or 2.25. For bonus points and much-needed speed, decrease the number of shrews and mount the handlers on dire wolves.
The shrew-pack is vulnerable to magical silence. On company scale, each time silence is cast on the shrew pack, it becomes disordered and must roll shock. Each silence in effect on a shrew pack increases their THAC0 by 1 and decreases their AC by 1, to a maximum of four points (so a shrew-pack that had somehow not broken while under the effect of four silence spells would have AC 2 and make 4 bites at 13+). On platoon scale, a single silence spell causes the full 4-point penalty, while on larger scales it would take more casting.
The shrew-pack is exactly the sort of center-line unit goblins need for melee fighting, and at a very reasonable price. Just be sure to not have anything behind it when it breaks morale.
And now for some "heroes". Naming courtesy of Dwarf Fortress. These heroes are really only viable for platoon-scale combat but that's OK, because that's typically the scale tribal goblins will be operating at.
Atu Helltwisted, Witchdoctor
Goblin Witchdoctor 6, Str 7, Int 17, Wis 14, Dex 13, Con 6, Cha 17
Class proficiencies: man I have no idea how class profs work for a hypothetical goblin class that doesn't exist yet... Intimidation seems reasonable though
General proficiencies: Healing 2, Alchemy
Equipment: leather armor +1, Potion of Fire Resistance, spear
HP 6, AC 4, init +1, THAC0 10+ melee for 1d6-1 (sad tombone.wav)
Leadership 6, ZOC 3, Strategic Ability +2, Morale Modifier +2
1/2/3 Foot Hero, no unit-scale attacks
2/day: Fireball, Dispel Magic (also Telepathy, Clairvoyance)
2/day: Stinking Cloud, Invisibility (also Deathless Minion, Locate Object)
2/day: Sleep, Shield (also Spider Climb, Choking Grip)
Atu Helltwisted is a competent leader and a dangerous spellcaster but very fragile physically. He probably leads from the rear except when casting. He can be used as an independent hero on company-scale because he has 3rd-level arcane spells.
Snamoz Murkyghost, Shaman
Goblin Shaman 8, Str 13, Int 11, Wis 15, Dex 15, Con 9, Cha 11
Class proficiencies: Beast Friendship
General proficiencies: Naturalism, Military Strategy
Equipment: leather armor, spear +1, Potion of Growth, dire wolf mount (Bramblewane, 25 HP)
HP 9, AC 3, init +1, THAC0 7+ for 1d8+3
Leadership 4, ZOC 2, Strategic Ability +2, Morale Modifier +0
2/5/7 Mounted Hero, no unit-scale attacks
Relevant spells (from the Shaman list):
1/day: Call Dragon, Insect Plague, Summon Weather
2/day: Dispel Magic, Skinchange
2/day: Call Lightning, Winged Flight
3/day: Bless, Obscuring Cloud
Snamoz Murkyghost is a mediocre leader and quite fragile, but Call Dragon is a good spell even on company-scale. If he's already used Call Dragon this week, Insect Plague is no slouch either. Skinchange, Winged Flight, and his wolf all give him substantial mobility.
Ago Thiefsliced, Redcap Captain
Goblin Subchieftain, Str 16, Int 9, Wis 9, Dex 18, Con 15, Cha 10
Class proficiencies: Skulking
General proficiencies: Leadership
Equipment: war scythe, leather armor +1, Potion of Invisibility, Elven Cloak
HP 10, AC 6, THAC0 7+ for 1d10+3, init +2, hide in shadows 9+, backstab x2
Leadership 5, ZOC 2, Strategic Ability +0, Morale Modifier +0
1/2/3 Foot Hero, no unit-scale attacks
Ago Thiefsliced is a pretty poor leader, but he can be dangerous to heroes when charging from ambush. He is probably best used as a lieutenant to reduce the activation cost of a platoon of redcaps hidden distant from the division's leader, though if you had three platoons of redcaps hidden together along a treeline, you could do worse for a captain. If you want a leader for redcap riders, swap Skulking for Riding.
Ngom Cradlethief, Goblin Chieftain
Goblin Chieftain, Str 14, Int 16, Wis 13, Dex 16, Con 11, Cha 13 (hell of a statblock)
Class proficiencies: Fighting Style (Missile), Command
General proficiencies: Riding, Leadership, Military Strategy
Equipment: lance, shortbow +1, leather armor, vampire warbat (Moonvexing, HP 12), potion of healing
HP 17, AC 5, THAC0 7+ melee for 1d10+3 lance or 4+ ranged for 1d6+2 shortbow (I'm assuming that as a 3HD underspecified monster-class, he gets 2 points of fighter damage bonus plus his 1 point of Str), init +2
Leadership 6, ZOC 3, Strategic Ability +3, Morale Modifier +3
3/6/9 Flying Hero, no unit-scale attacks
Ngom Cradlethief is old, cunning, and vicious. He is a very competent commander at platoon scale, and his giant vampire bat mount has mortally surprised more than one human lieutenant. In a platoon-scale fight against a 4th-5th level PC party with mercs, he should probably try to avoid directly attacking heroes (fireball and an archer or two would make that a short engagement) and seek to win the battle by leadership and hopefully numbers, where he is strong.