Cerberus

 was a three-headed dog that guarded the path leading from the river in the Underworld in Greek mythology.

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)