Cerberus - Greek Mythology

Age of Fable
"Hercules descended into Hades, accompanied by Mercury and Minerva. He obtained permission from Pluto to carry Cerberus to the upper air, provided he could do it without the use of weapons; and in spite of the monster’s struggling, he seized him, held him fast, and carried him to Eurystheus, and afterwards brought him back again."

Cerberus - The Devil's Dictionary
"The watch-dog of Hades, whose duty it was to guard the entrance--against whom or what does not clearly appear; everybody, sooner or later, had to go there, and nobody wanted to carry off the entrance."

The Divine Comedy: Hell
Dante writes: "Cerberus, cruel monster, fierce and strange, / Through his wide threefold throat barks as a dog / Over the multitude immers'd beneath."