Mythology & Beliefs: Cerberus In Greek and Roman Mythology, Cerberus was the three-headed dog guarding entrance to Hades.
Cerberus in Greek and Roman Biography and Mythology
（*Ke/rberos), the many-headed dog that guarded the entrance
of Hades, is mentioned as early as the Homeric poems, but
simply as " the dog," and without the name of Cerberus. (Il.
8.368, Od. 11.623.) Hesiod, who is the first that gives his
name and origin, calls him (Theog. 311) fifty-headed and a son
of Typhaon and Echidna. Later writers describe him as a
monster with only three heads, with the tail of a serpent and
a mane consisting of the heads of various snakes. (Apollod.
2.5.12; Eurip. Here. fur. 24, 611; Verg. A. 6.417; Ov. Met.
4.449.) Some poets again call him many-headed or hundred-
headed. (Hor. Carm. 2.13. 34; Tzetz. ad Lycoph. 678; Senec.
Here. fur. 784.) The place where Cerberus kept watch was
according to some at the mouth of the Acheron, and according
to others at the gates of Hades, into which he admitted the
shades, but never let them out again. - A Dictionary of Greek
and Roman biography and mythology, William Smith, Ed.
Cerberus in Wikipedia
Cerberus, (pronounced /ˈsɜrb(ə)rəs/); Greek form: Κέρβερος,
/ˈkerberos/ in Greek and Roman mythology, is a multi-headed
hound (usually three-headed) which guards the gates
of Hades, to prevent those who have crossed the river Styx
from ever escaping. Cerberus featured in many works of ancient
Greek and Roman literature and in works of both ancient and
modern art and architecture, although, the depiction and
background surrounding Cerberus often differed across various
works by different authors of the era. The most notable
difference is the number of its heads: Most sources describe
or depict three heads; others show it with two or even just
one; a smaller number of sources show a variable number,
sometimes as many as 50...