The Lernean Hydra in Heracles (Hercules): The Twelve Labors
The Hydra is a huge venomous snake with anywhere from nine to a hundred heads, depending on which version of the myth you're reading. With the help of Iolaus, Heracles kills this awful beast as his second labor. It is pretty tough because every time you chop off one head, two more sprout in its place. However, Iolaus cauterizes the stubs after Heracles chops the head, and eventually the monster is destroyed. Heracles dips his arrows in the venom of the dead snake to use as deadly weapons.