Pitbullfrogs were squat predators. As their name suggested, they looked like a cross between a pit bull and a bull frog. They had warty, grey skin, gangly legs and a wide mouth filled with shark-like teeth. They were extremely hardy, capable of surviving several gunshot wounds.
When Kerko and his brother came to Cardiff, they were able obtain two pitbullfrogs. When Jack Harkness came to capture them, they tried to get the pitbullfrogs to attack him. He was able to kill one, while the other one escaped. This pitbullfrog was spotted by the police. It killed the police dogs sent to restrain it. The pitbullfrog eventually encountered Ianto Jones and Gwen Cooper, who shot it. (PROSE: The Undertaker's Gift)