In the mid 21st century, Captain Grant Matthews was one of only two humans that the Myloki duplicated, creating an indestructible version. The Second Doctor believed the copy was more of a Myloki-human hybrid since he seemed to be exactly like the original. Karl Taylor, on the other hand, was more like an animated corpse.
His Myloki programming was reversed, leaving him with free will. Decades later, the Myloki tried to retrieve him so they could sever their ties with our reality. The Doctor convinced him to go to them so he might find some happiness, since he had come to hate his immortality. (PROSE: The Indestructible Man)