It has three heads, and for every head you cut off, two more grow in its place.
the hydra
It was a hydra.
It is a hydra
hydra
The monster you are referring to is the Hydra. The Hydra is a child of Typhon and Echida.
Greek mythology has no chronology in our sense.
You may be referring to the Hydra, but the hydra was also said to have nine heads instead of six.
The "Hydra" but dont let that fool you, hydras can have 5+ heads.
The nine headed creature was a hydra
Poseidon was the Olympian god of the sea. There are numerous other, minor water gods such as Asopus (Ασωπός).
Lernaean Hydra.
Greek ironclad Hydra ended in 1918.