Spore

An excellent simulator of an evolving creature

Pros

+ A bunch of different stages of evolution, each stage has its own mechanics

+ Character editor that allows you to make the character whatever you want. That is, really any way. You can attach its legs to its back, its mouth to its tail, and its eyes to its subspecies (when you decide what exactly it is), and at the same time bend the creature with the letter zy

+ a bunch of different details for the editor

I highly recommend