Soto is an alien from another planet whose cellular structure is composed of living stone. He originally owned a dog, but the dog was far more intelligent and cultured than his master, who in turn was very childish. No longer willing to remain with Soto, who did not appreciate his intelligence and free will, the Dog escaped to Earth in an escape pod, but was relentlessly tracked by Soto and his ship.
During his flight, the Dog accidentally ran into Beast Boy, who was kidnapped in his stead. Soto lost no time clapping Beast Boy into a collar which would keep him captive no matter what shape he assumed. Excited by Beast Boy's wide range of possible shapes, Soto instantly decided to keep Beast Boy as his new pet and take him back to his home planet, disregarding the latter's protests to the contrary.
In the meantime, the other Titans, who had mistaken the Dog for Beast Boy, learned the truth about their quarry, who offered his help in finding Beast Boy. They managed to prevent Soto from taking off, but Soto himself proved to be too strong and resilient even for their combined powers. But then Beast Boy, freed of the collar, appeared on the scene and soundly thrashed Soto, and the collar ended up on Soto's neck instead by his own Dog's tongue. When the Dog refused to submit to the role of the pet anymore, Soto enthusiastically adopted the role of the pet for himself, and Soto and his new master left the Earth, leaving Beast Boy to reunite with his teammates.