The Goat is the default evolution.
The Goat is the main character in the game as the name implies it is the first mutation you spawn in the game with. It allows you to lick people/objects, head-butt things, and just generally be a goat. It resembles a myotonic wether or neutered male Fainting goat (hence the ability to "ragdoll").
The Goat has many mutations you can find around the two maps such as