I am Bread is a mutator that turns the goat into a slice of bread from the game of the same name.
See the I Am Bread Achievement.
Alternatively, here's a video showing how to unlock it:
- This is a reference to the video game "I am Bread", which also had some Goat Simulator references in it for a little while.
- The I Am Bread goat is unlocked as a result of completing the achievement "I Am Bread".