Or, I spent 15 hours reading wikipedia pages on venomous snakes so I guess I may as well make a post out of it.
I am not a herpetologist. I am not your herpetologist. Nothing in this post should be construed to be medical advice, nor expected to be perfectly accurate. Everything here is gross generalizations for the purposes of gaming.
The two big families of venomous snakes dangerous to man are the elapids and the vipers (there are some dangerous ones in other families like the colubrid boomslang though)
- Include cobras, taipans, sea snakes, coral snakes, kraits, mambas, pretty much all of the various intensely venomous Australian snakes with unassuming names like the Western Brown Snake...
- Often relatively long and thin body form
- Mostly have round pupils
- Mostly lay eggs
- Have relatively short, non-folding fangs at the front of the mouth
- Venom is often primarily neurotoxic and kills by stopping respiration
- In some cases, venom is almost entirely neurotoxic in action and causes no pain or swelling at the bite site, making it hard or impossible to tell if a bite was "dry" until onset of symptoms
- The combination of short fangs and quick-acting venoms often lead to an attack pattern against their primary prey of wrapping around and biting multiple times to guarantee some good deep killing envenomations
- Hunting pattern is often active - seeking out prey, going into burrows
- In humans, time to kill from a wet bite is often 30 minutes to six hours
- Include rattlesnakes, copperheads, cottonmouths, adders, pit vipers, bushmasters, fer-de-lance, ...
- Pit vipers are called this because they have a pit on each side of their
head between the eye and the nostril which can sense infrared, not because they live in pits. It's actually a pretty big category and includes most (all?) New World vipers
- Often relatively stocky/girthy/thicc body form
- Many have slit-pupils like cats
- Mostly give birth to live young
- Have long, thin fangs that fold up against the roof of the mouth when not in use, and lower jaws that hinge out past 150 degrees to let them strike with the long fangs
- Venom is often primarily toxic to blood and muscle tissue, causing clotting, hemorrhage, blistering, necrosis, kidney failure from rhabdomyolysis. May require amputation of the bitten limb even if it doesn't kill you.
- I have now seen some pictures of necrotized viper bites that I cannot unsee
- Wet bites are typically very painful and swell up
- Between the fragile but long-reach and deep-injecting fangs and the slow venoms, attack pattern against primary prey is often a single lunging bite and then backing off and waiting for the prey to die.
- Some can track bitten prey by the smell of some components of the venom acting on the prey's blood
- Often a passive hunting style, waiting in ambush for passing prey to tag
- In humans, time to kill from a wet bite is often 10+ hours (overnight or the next day) unless the bite was onto a vein
On reflection, it seems like regular-sized mundane venomous snakes are really more like traps than they are combat encounters. You didn't poke the pile of leaves with a 10' pole before you stepped in it, save vs poison, and unless you roll a 1 you still have at least half an hour to get a Delay Poison or Neutralize Poison in before you keel over. The necrosis angle on viper bites could be somewhat interesting, might play out a bit like ACKS' Dismember spell on a failed save. 2HD for a 5' pit viper that probably weighs 5 pounds seems really high.
Where you'd expect to see save-or-die poison with a pretty quick time to kill would be in snakes for whom humans are a common prey species. I recall reading somewhere that most predators hunt prey that is something like a 10th of their own mass to minimize the risk of being injured by the prey. I don't know how true this is but it sort of passes the smell-check; a mouse is much smaller than a cat, a mosquito is much smaller than a bat or sparrow, a seal is much smaller than a great white shark. I would expect pack hunting, ambush, and venom to all shift those closer to 1:1, since these strategies reduce the risk of injury to the predator.
The Eastern Diamondback Rattlesnake is typically about 6 feet long and weighs about 5 pounds. It commonly preys on cottontail and marsh rabbits, which weigh about 2-3 pounds. Scaling up mass with the cube of any single dimension, we'd expect a 12 foot rattlesnake built to the Eastern Diamondback's proportions to weight about 40lbs, and a 24-foot rattlesnake to weigh about 320. That seems a size at which hunting adult humans as a primary prey sounds plausible. Incidentally, this is also about the size of a large reticulated python, which have been known from time to time to prey on humans, including adult male humans. I think this length and mass would be a better match for the 4HD giant rattler stats than the 10' in its description.
Anyway, a few other fun snake "facts":
- King cobras have a really low-pitched growly hiss apparently
- The yellow-bellied sea snake can get about 33% of its oxygen needs by absorbing oxygen from the water through its skin
- We're pretty sure sea snakes don't drink seawater, but nobody is really sure where they get fresh/brackish water. It's theorized that they might drink the layer of brackish water at the top of the water column during heavy rains
- The small-scaled burrowing asp can rotate its fangs sideways out of its mouth and uses this in confined spaces where it doesn't have room to bite. It has also been observed to sting each rodent in a burrow containing multiple before stopping to eat any of them. Next time your players meet a snake and complain that trying to bite each PC in turn was too smart, show them this.
- Some venomous snakes which hunt by ambush use "caudal lures", where the tip of their tail look like a tasty worm or grub. A dungeon-snake whose tail looks like some sort of unattended treasure would be pretty funny.
I also wandered into some articles on treating snakebite. Antivenin is made by injecting large mammals like horses with small doses of venom and then harvesting their antibodies. Antivenom can have some pretty significant side effects, called "serum sickness", from reactions to horse proteins. These can take up to two weeks to appear and in rare cases can kill you. In folk medicine, there's a whole genre of magic healing stones, some of which nominally work on snakebites (bezoar stones from inside of toads and snake-stones or black-stones often made from burnt animal bones) and some of which might be made by snakes (adder stones). There were also madstones, which might not have been stones at all but body parts of albino deer used to try to treat rabies? In conclusion I feel OK about having some non-magical treatments for snakebite that give you a second save but also entail bed rest afterwards.