A hydra has a bulky body and no fewer than eight heads at the end of as many serpentine necks. The draconic heads are roughly the size of coffins, but the heads differ slightly in size and appearance. There is always one head that governs the others. In order to defeat a hydra, one should confuse and distract the other heads and then attack the governing head.
Be warned: every time a head os severed, two new ones will grow back.
There is an ancient hydra called Hespera, one of the 3 guardians of the Dragon Temple in Wyrmroost. Hespera was killed by Gavin Rose, a.k.a. Navarog the Demon Prince, during the quest to get the key to the Translocator in Book 4.