A hydra is a monster introduced in SLASH'EM, one of two non-dragons to use the D glyph, the other being the wyvern. True to its source mythos, it has a large number of bite attacks. It cannot, however, grow new heads when attacked with a sword.
These should not pose a problem by the time one encounters them; they are somewhat slow, do only a moderate amount of damage, and have no particular special attacks. They do have a large amount of hit points and a decent AC, though, in addition to resisting stoning.
The hydra is of course the monster from Greek mythology. It's full name being the Lernaean Hydra (Λερναῖα Ὕδρα). It is a large, snake-like monster, with multiple heads, that Heracles fought, which would grow two new heads for every one chopped off.
It may also be a reference to the same monster in Crawl, which also uses the D glyph. It is one of the more notable monsters in Crawl, as it does have the ability to regrow heads, and will do so whenever attacked with a non-flaming edged weapon.