The Zimon was once a timelord but over years of feeding off time energy he transformed into the beast above. As a child he hated the Doctor and the Master, whilst others looked up to the pair. At 90 he tried to push the doctor into the mutters spiral, at which point the doctor fed him his first part of time energy. The Doctor later blamed himself for this. At 214 the Zimon became part of the High Council of Timelords. This lasted only a month, because he was going mad and transforming fast.By 256 he had transformed as much as he was going too and forgotten his original life as a timelord. All he knew was that he loathed the Doctor. He set off into the vortex.
400 years later he arrives on earth, knowing that a woman called Winifred Stough has a connection with the Doctor. He chases her but is stopped by the Doctor, who makes time energy poisonous. But fatally, the Zimon has no understanding of this concept, and sets off on his own quest (S1: The Trekk of the Zimon)
5 years later he dies of starvation.