Poison Spiders (otherwise known as Green Spiders) appear in the Spelunky X mod as one of the 3 poisonous enemies in the game. It is an advanced version of the Spider as it can poison you, move quicker, and spawn from pots more commonly.
The Poison Spider was originated from the original, nonvenomous spider from all versions of Spelunky. It was modeled to become a Spider, yet Green and harder to spot.
Unlike normal Spiders, they can't be added in custom levels, unless they spawn out of pots, or you put there object in the level maker file in Game Maker. Also unlike normal spiders, thankfully, there is no giant Poison Spider.
The Poison Spider is mainly in the Jungle Levels, but in all other levels (except the final level, unless rarely spawned by the Giant Head) since they also spawn out of pots, and is very hard to spot, and once under, it immediately falls on you. Not only is it harder to spot, it also falls 2x faster than normal spiders, making it difficult to whip it while it falls. Not only does the green camouflage it's environment, but it only shows it's yellow eyes on the top of a jungle cave, making it not imposable to find.
There is a difficult way to kill poison spiders, though it will tick off some people speed running Spelunky. The only way to kill a green spider is to throw a rock at it, jump on it, or lead it to water, since spiders can't swim. You can also lead spiders from pots or custom levels to water. Once in water, it will blink blue and drown, but beware, it will still hurt you in water, and you can safely whip it for bonus xp. You can also whip Poison Spiders, but it's sort of hard, considering it would probably bite you and poison you first before your whip will hit it.
Be on the look out for Poison Spiders on Level 5 and Restless Dead Levels the most, since they are most common there. They are also sort of common under the slope of land above the platforms on Rushing Water Levels. So, be careful when walking there.