Minotaur.
The Minotaur, who was the son of a white bull, and the queen of the city of Crete. It lived in a labyrinth in the city and fed on sacrifices of seven boys and seven girls.
A six-headed monster is often referred to as a "hexapod" in a general sense, but in mythology and literature, it might not have a specific name. In Greek mythology, a well-known multi-headed creature is the Hydra, though it typically has many heads rather than a fixed number. A six-headed creature could also be creatively named or referred to in various fictional works, depending on the context.
Cerberus is the name given to the entity which, in Greek and Roman mythology, is a multi-headed dog which guards the gates of Hades, to prevent those who have crossed the river Styx from ever escaping.
Scylla.
The 3-headed dragon in Percy Jackson and the Lightning Thief is called "Chimera". It is a monster from Greek mythology with a lion's head, a goat's body, and a serpent's tail.
Literally, "bull-headed", or stubborn...
Poseidon was the Olympian god of the sea. There are numerous other, minor water gods such as Asopus (Ασωπός).
Cerebus/Cerberus
Cetus is a constellation named after the sea monster from Greek mythology. It represents the sea monster that was sent by Poseidon to devour Andromeda. The name Cetus is Latin for "whale" or "sea monster".
Cerberus was the three headed dog that guarded the entrance to the underworld.
Five-Headed Dragon.
The monster with a hundred hands, in Greek mythology, was called Hechtonchires.