Jack was a 21st century human.
He and his friend Shell were trying to stop animal experimentation. They convinced Ace to help them investigate one site, but they were caught. To avoid arrest, Sean made them agree to take warlock. The drug transferred their minds into animals nearby. Shell was killed, Ace's mind was restored to her body, and the dog with Jack's mind escaped. The Seventh Doctor kept Jack's comatose body in a life support module, hoping to eventually restore Jack's mind to his body. (PROSE: Warlock)
Years later, the Doctor found the dog, which had become known as the White King. He used some warlock he had saved to put Jack's mind back into his original body. (PROSE: Warchild)