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).
To open the GUI of the Morph mod simply press the M in the bottom right corner of the screen. From the new menu which opens you can choose among a large list of mobs to turn into.
There are all different kinds of mobs you can turn into. Some examples are chicken, cow, mooshroom and a pig.
Then you can also turn into humanoids like the villagers.