I can create a new creature actor in the CK, and get it to follow me exclusively with AI Packages. How can I get the creature to attack enemies too? I tried adding a relationship but it doesn't seem like one can be created between the player and a npc (the actor doesn't show up in the drop-down menu).

anyone?

Look at how work the human followers. Then do the same. Or better, duplicate a follower and change its model to the creature you want.

Or try to download some mod with creature follower, and look out how it's done.