Cerberus was a vicious Hellhound that would guard the Underworld, allowing the dead to enter, but never leave. According to myths, Cerberus was a mix of creatures combined: three wild dog heads, dragon/serpent tail, and snake heads on its back. 

Echinda(half serpent and woman) and Typhon (a giant that was covered in dragons and serpents, who breathed fire) were the parents of Cerberus. Finding and catching Cerberus was also Hercules' last labor. 

"...A monster not to be overcome and that may not be described, Cerberus who eats raw flesh, the brazen-voiced hound of Hades, fifty-headed, relentless and strong."

- Hesiod, Theogony 310