From Destinypedia, the Destiny wiki
These Hydras are seen as the second bosses of both the Gauntlet and Crystals encounters, where they attempt to inhibit the Guardian Fireteam's progress through the Menagerie. They are to be killed in order to progress through the encounters (such as spawning new Crystals or beginning a new round of the Gauntlet), and gather points. They will remain in their positions (atop the middle altar in the Gauntlet or at the top of the stairs in the Crystals), where they lay into the team with their Aeon Maul and use their Temporal Shockwave if any get too close.
List of Appearances