Gus is a mutant bulldog who is loyal to Big Mama.
He first appeared in "Bug Busters" where he is sent by Big Mama to hunt down Leonardo. After Leonardo rescued Raphael, Donatello, and Michelangelo before they can be used as rodeo clowns in the Battle Nexus, they are attacked by Gus. Michelangelo uses a ball to distract Gus where he threw it down the hall. Gus caught up to the Turtles at Big Mama's office reminding Michelangelo that he should never start a game of fetch that he can't finish. When Baron Draxum has his minions Huginn and Muninn target the jar of Oozesquitoes, Gus attacks them. During Baron Draxum's fight with Big Mama, Gus sees the ball rolling into the vault and follows it.
Gus can manifest spikes on his body. However, he is prone to partaking in a game of fetch.