hydra
The Lernaean Hydra. Child of Typhon and Echidna. Killed by Hercules.
The monster you are referring to is the Hydra. The Hydra is a child of Typhon and Echida.
A snake in a dream usually represents a fear, maybe cutting off the head of this huge snake represents that you have or will overcome one of your greatest fears or worries.
It depends on the size of the snake. If the snake is small, the head will be small. If the snake is big, the head will be big.