I have Minecraft Forge installed. I took a look at some source code files, including EntityCreature.java, and could not find the code that makes the creatures wander around. I want to adjust the code to allow the creatures in the game to fly.
Look into the EntityFlying class, or the Ghast as a complete example
Might be as simple as making the creatures you want to fly extend EntityFlying and implement IMob
Probably not though as this will probably make them behave in a strange manner