|Monica C. Webster|
"Hercules and the Hydra"
Everybody knows of the Lernaean hydra. There are 5, 6, 7, 8, 9, and 10 headed hydras. There are pyrohydras and cryohydras. And they can have any number of heads above. All of this led the players to speculate about what type of hydra would be hiding in the Forge District. It turned out to be a pyro-hydra, and slightly different from the AD&D version. Namely, each head could breathe fire 1/day for 5d6 damage.
This just further illustrates my point. Hydra are hard to quantify in a PCs mind. They can be a hundred different mutations "by the book." And even more when you let your mind wander.
Moreover, the hydra is a fairly rare creature in modules. According to Adventure Lookup they have appeared in 34 adventures. TSR had 14 modules, WotC has 4. They are severely underused.
How best to use a hydra, though?
In the Palneal Swamp, it is reputed that a 5-headed hydra hunts the southern edge. Local fisherman have been seeing it closer and closer to the river. The fishermen are panicked and refuse to get near that stretch of river. The local lord asks heroes to deal with the imminent set.
Deep on the trade route between Redstone and Chattin a hydra has buried itself within the sands. It only attacks with its heads, and keeps its body below ground (giving it boosted AC.)
A sect of kobolds within Mord Mar worships a hydra near an underground lake. They often lure adventurers near its lair, and let it feast.