"A little lizard with a big appetite. Rex has decreased max health but drains health from enemies."
Rex is unlocked from finding a green egg randomly generated in a level, pressing the "e" key to pick it up, and left clicking it to throw it. He will then be available to play as next time you go to the character select screen. He starts with 50 max health and will receive 1 HP when hitting an enemy.
Rex also has a random chance to appear yellow when selected as shown on the right. Strangely, when yellow Rex dies his animation shows green Rex. This could be a graphical bug or a hastily done animation, accidentally showing green Rex.