The hydra is a many-headed serpent.
(As in: the heads each have a serpentine "neck," a opposed to just having a snake with a bundle of heads on one end.)
It's big and poisonous.
Maybe it's aggressive, or maybe ancient Greek heroes just liked to
get into fights with dangerous animals.
If you cut off a head, it head grows back. Or a pair grows back.
Much is made of Heracles' solution - cut off a head, and cauterize the stump immediately.
But why not strike at the body?