Greek Monster

You are probably looking at that picture and wondering what that is. This is Cerberus, the three-headed demonic dog who guards that gate to Hades, or Hell, your pick. Because he is the watch dog, Cerberus doesn't go out much to wreck havoc in human race. Possibly, the only time he went out was when Hercules had to get him out for his Twelve Tasks. Hades keeps Cerberus to keep the living out, to keep the dead in and to permit more dead from Charon, the ferryman. Hey if you don't know, look it up for yourself.
Cerberus is supposedly the offspring of two Titans, Echidna and Typhon, both monsters. Some of Cerberus's siblings are the Nemean Lion, Chimera, the Lernaean Hydra and the Scylla.
There are several times when mortal living humans passed by Cerberus in order to get into Hades for various reasons. For example, Orpheus put Cerberus to sleep with his music, and Psyche used honeycakes(...) go get by. And there is Hercules, as mentioned before.