Hydra heads are nimble and strong, and even one by itself can do significant damage with its jaws. Some of the larger varieties, sporting upwards of ten heads, can easily face down a group of seasoned adventurers by itself. Even more terrifying is the matter of truly killing or even subduing one of these monsters - Hydra possess remarkably fast healing, making attacking the body fairly futile. Their long, slender necks might seem an excellent target, and in truth a Hydra can be slain by severing each one in turn - but the Hydra's fast healing extends to these wounds too, and a severed stump will regrow two new heads in 1d4 rounds unless cauterised with fire or acid. Fighting a Hydra effectively requires much planning and teamwork - but at least you end up with a good selection of trophies!
Another "classic" creature down! I had fun drawing this guy. As I was checking out the stats for Hydra in 3.5 they actually seem like kind of a good option for newer adventurers who want to fight something big - even the five-headed variant is in the Huge size category, making for an impressive fight, but the challenge rating isn't too high and besides the nasty bite attacks (5 x 1d10+3 damage potentially, ouch) they're not too scary, stat-wise. Provided you know about the whole "head regeneration" thing, anyway!