The hydra is a monsterous reptile with many heads
Most mythes agree that the hydra is a sea monster with up to one hundred heads and one immortal head that can not be harmed by any weapon in the center of it's group of heads. he has webbed, feet a serpent tail and a thick, strong body.
It grows 2 heads when one is cut off. If the neck is burned, two new heads will not grow. This was shown in the myth of Hercules fighting the Hydra.
Some also say that it breathes poison. Its blood is also poisonous. The largest head is an Immortal head. It can not be truly killed.