Billy the Orca is a killer whale who first appears in the episode "SpongeGuard on Duty." In the episode, SpongeBob is seeing some "dangerous" things at Goo Lagoon, which was featuring Billy. SpongeBob actually thought Billy was going to eat the kids playing ball in his mouth. But Billy was actually giving the kids a ride while they were playing ball.
Billy is also seen in a level of Marbles & Slides.