The Cerunnos

The Cerunnos was monster spoken of in the myths of planets across the universe and which was believed to have existed in the Dawn Time.

Biology
The Cerunnos was parasitic and could assume any form. The Doctor considered it madness made flesh. (VD: ...And Eternity in an Hour)

History
The Tonska inadvertently called forth the Cerunnos by exposing a volunteer to the energies of a time rift. (VD: ...And Eternity in an Hour)