The Hydra is a multi-headed dragon-like beast that lives in Equestria.
One of the few monsters to be found outside the Everfree Forest, the Hydra makes it's lair under swamps. Capable of lurking underwater it waits for suitable prey, then ambushes it when it's back is turned. It also proves rather fast on land too, though it has some trouble turning at speed.
It has four heads, each of which seems to think independently. It is these that are it's most dangerous weapon, they are capable of extending with considerable speed and force, and can attack in multiple directions. It's normal method for attack is to try and butt it's targets, squashing them against the ground.
The term Hydra is actually a description, the beast of legend is actually unnamed. Properly called the Lernaean Hydra, it was a water dwelling beast of many heads, which would sprout two new ones each time one was cut off. A guardian of one the gateways to the underworld, it was raised by Hera to kill Heracles and he was forced to fight it as one of his twelve tasks. Hercules found out that the only way one could kill the Hydra was by burning the head off.