We’ve had a run of new (that is, late-era D&D or Pathfinder-only) monsters recently, with more on the way (here come the ice linnorms!). But the hydra is as old school as it gets. Pathfinder takes the hydra back to its roots by ditching the distinction between regular and regenerating (Lernaean) hydras. With fast healing and heads that regenerate to double the original, these are nasty beasts that are hard to kill.
And that’s appropriate. After all, the original hydra killed even after death—its blood fouled an entire river, served as the poison for Heracles’s arrows, and would up killing the demigod himself (in the form of a poisoned cloak). Your hydra will probably only rarely be the boss monster in an adventure, but it should be hard to kill. Give it lots of opportunities to escape and live to regenerate again—and if it does, take it as a given that it will strike the PCs or someone they care about soon. A hydra may not be the reason PCs enter the swamps, but it can be the reason they never leave.
A hydra has never forgotten the young black dragon that stole its lair, drove it downriver, and left it with acid-seared stumps where two of its heads used to be. Now it attacks random villages, gorging itself until hunters are sent after it. These it leads upriver to the dragon’s domain. The cagey hydra intends to continue this way until the dragon is finally slain and it can return to its beloved lair.
Tales of an unwatched clutch of white dragons bring fortune seekers running to the ice caves near Shelter, hoping to catch and sell the wyrmlings before they’ve imprinted. What they find is icy death. A cryohydra hides in the semi frozen pools, using one head to draw out would-be hunters, then attacking from behind with the other five.
Pyrohydras are rightly feared in mountain communities—especially in the summer months, when a single spark can set the tinder-dry trees aflame for leagues in every direction. But when a pyrohydra is spotted on Carter Peak, the local lord doesn’t want it slain—he wants it caught. Ostensibly it is to be sent as a gift to a neighboring fire giant thane as a peace offering. Really, it is a bribe to get the fire giants to attack the lord’s rivals. He finds it a delicious irony that the adventurers he’s hired don’t realize they are helping him incinerate their hometown.
—Pathfinder Bestiary 178
Speaking of monsters coming back to haunt you: Two years ago, my comic book store got ahold of a ton of old metal miniatures and gave them away on Free RPG day. Not being a mini guy, a picked the hydra and gave it to my GM. Lo and behold, it came back to haunt us, in the form of a pyrohydra that had even my fire-resistant paladin running for cover. It went on to slay the bear animal companion that practically defined our taciturn druid. He was not pleased.
2e fans should check out Jonathan Richards’s “The Ecology of the Hydra: Heads and Tales” from Dragon 272, one of the very last pre-3.0 Dragon Ecologies.
In Mythological Monsters Revisited, Jason Nelson has some nice thoughts on hydras, especially how to make them fearsome combatants in their home environments. I’m mixed about his suggestion that they breed by asexual budding, though. That seems a poor strategy for genetic diversity and pyro/cryohydras are far more likely to have arisen through red/white dragon crossbreeding than mutations alone. I’d argue that hydras mate when they can, and then bud during times of environmental stress.