Cerberus

Cerberus was a three-headed dog with a tail made of snakes. Cerberus guarded the path leading from the river in the Underworld (or "Hades") in Greek mythology.

Biography
Hercules was tasked with capturing Cerberus as one of his impossible Twelve Labours. When he succeeded, the terrified Greek king immediately ordered Herucles to put the dog back where he'd found him. (PROSE: Mythical Monsters)

When Katarina and the First Doctor approached Cerberus, it would not let them pass. Charon explained that it would only let Katarina pass, as the Doctor was not dead. She refused to leave the Doctor. He fashioned a dog-whistle and contrived to have it whistle continuously. While the beast was distracted, they made their way past it. (PROSE: Katarina in the Underworld)

Alistair Gordon Lethbridge-Stewart encountered Cerberus in the Greek Underworld. (PROSE: Deadly Reunion)

On Augea, the Doctor and Hercules had to muck out Cerberus after cleaning out the Augean stables, which was a more difficult task. (PROSE: Sting of the Zygons)

Legacy
A Cerberus (with two fake heads) was one of the attractions at the Ancient Worlds theme park on Dewyn. It came under the control of a Chiffala and was used to attack guests attending the park. (COMIC: Minor Trouble / Inhuman Sacrifice / Crimes and Punishment)