It will take a little of a challenge since you have to kill mob to turn into it.
Morph Victim Mod
With this mod you can quite easily turn into any mob you stumble upon. It will take a little of a challenge since you have to kill the mob to turn into it.
Morph Mod
The Morph mod adds a new GUI to the game which can be used to turn into any kind of mob you like (except for slimes and wolves).