This is about the anime characters. For the monster itself, see Usaba.
The Usabas are one-eyed onion-like monsters who work in the prison that Pixie has set up in the ruins near North Town. They make up the body of her home troops, much like the Sandys do for Gali and the Cabaloses do for Gray Wolf.
Presumably she keeps them around because their large eyes allow them to watch the slaves closely, but in all actuality they are pretty dim-witted, and Genki outsmarts them pretty easily, allowing him to gain access to the prison and free the prisoners. The Usabas then join forces with the Clays and Black Dinos to keep everyone from escaping, but they disband when Pixie gives up.