MisterE Cave spiders drop a lot of meat, web, and the web can be made into silk (which is currently useless). They will mostly leave you alone if you can avoid them, but if you get too close they will shoot a webball onto you and trap you. Then they'll move in for the kill! They can also climb really well.
In the future I hope to learn to add poison to mobs. I would then change the spider to have a weaker bite, but have poisonous effects, as well as some other mobs.
Crocs cannot pull people down, that would be great. I wish I knew how to add that. My last attempt at adding entity's that attached to players did not work well, it was the alien facehugger.