I would think for a catterpillar you could start out with x amount of stitches and then slowly decrease to a certain point and then increase back to the amount of stitches that you started with. This should give you those catterpillar bumps. Just repeat until Mr. Catterpillar is the length you want. Then I-cord some antennaes and sew on a pair of eyes and a smile. I'll keep looking for a real pattern though. Hope that helps :D .
I would keep an eye on this site:
It is pretty recent and has a very cute catterpillar on it. The person who made it said they will post the pattern soon. Maybe that will give you some ideas on how to make one. :D
Ohhhhhh !!!! How about this one ????
The only thing you have to crochet is the antenna and I could think you could I-cord that if you can't crochet (like me lol).
Woops that one was already posted ==== hehe =====
Ok this one is just like the one above, but a little different.