The Hydra is a multi-headed demon often used as a guardian.
Hydras are medium-sized, vicious, unintelligent, and blind, and thus rely on sound, scent, and their sheer number of large mouths to catch their prey.
Hydras have dark green flesh, thick black tongues, and enormous gaping jaws from which green drool drips. They have at least three heads, but often have more. They are distinguished from other multi-headed demons by the fact that their heads are mounted on necks that look like thick, swaying stalks.
A three-headed Hydra was guarding the Church of Talto in 2007 when Clary Fray came in to investigate. Clary struck one of the Hydra's necks with the cult's athame, but it quickly healed from the non-lethal blow as the blade was un-Marked. With her stele, Clary Marked the athame with an enkeli rune, dissolving its occult symbols, and threw the dagger at the demon, striking the middle head deep and right in the center of the skull.
The middle head simply dropped, but the demon remained intact, dragging the dead head along with it. Clary tried to escape, to no avail as the fast demon blocked her way out. The two remaining heads attempted to strike at her but were stopped by Isabelle Lightwood, who lassoed the heads with her whip, cutting both off instantly. The headless demon promptly disappeared, back to its home dimension.