Help with a 2 stage mob?

Hey all, I'm going to be working on a 2 stage mob which is basically a human who transforms into a werewolf, I've never made a 2 stage mob before and was wondering can someone point me in the right direction on how to pull this off? I looked through the manual and didn't see anything. I remember seeing something about it once but I honestly can't remember where. Thanks in advance!

Edit: I just noticed I posted this in the tutorial section instead of support sorry can it be moved?