Buster was a Pit bull (type not specified) with rabies that bit Marco in book 42. Marco referred to him as "Cujo" before discovering his name. Marco is forced to morph in order to hide from Buster's owner, and in the process destroys the virus that might otherwise have killed him.
Tjala is Yami's dog. Cassie describes him as "a sturdy little dog, not more than a half-grown pup, with dark speckles all over his coat and sharp ears."
"Wuss" is a male Helmacron who serves as a teacher for Cassie and Marco when they are captured by the Helmacron ship. His nickname is derived from their first conversation; Cassie asks for a name, but the Helmacron does not know how to answer (implying that Helmacrons, or at least the oppressed male Helmacrons, don't have names), so Marco starts calling him a wuss. He shows them that the Helmacrons have killed all high-ranking officers because they cannot trust them. The logic in that is that a captain (or anyone else in a high-ranking position) would inevitably make several mistakes, and since they cannot allow that, all Helmacrons have to be dead before they can rise to a position of power.