Hydra
Hydras are multi-headed dragon-like monsters originating from Greco-Roman Mythology. Described as a many-headed, poison-spewing serpent that could regrow its heads as fast as Hercules hewed them off, it was natural that such an iconic monster would go on to appear in other fantasy games.
D&D Hydras
The hydra has been around in Dungeons & Dragons literally since the beginning, having appeared in the Basic Set in 1977 and then going on to appear in the 1981 and 1983 Expert Sets, the 1991 Rules Cyclopedia, and then every Monster Manual from 1st through to 5th. Whilst various details have changed across the editions, the core has remained; hydras are quadrupedal, wingless dragon-kin who possess multiple heads on long, serpentine necks. They can regenerate, and favor swampy environments.
Traditional variants of the hydra include the flame-spewing Pyrohydra, and the frost-spewing Cryohydra.
Warhammer Hydras
Hydras have been a mainstay monstrous unit of the Dark Elves of Warhammer Fantasy. They are extremely tough and vicious creatures that ca regenerate and spew fire, but also stupid creatures that depend on Druchii beastmasters to keep them under control.