From Destinypedia, the Destiny wiki
The Taken Hydra is a Taken Vex that keeps its Aeon Maul but loses the Hydra's protective semi-circular shield, making it vulnerable to ranged attack and reliant on its support troops for overall protection. It also detonates if a Guardian gets to close, much like The Undying Mind. Certain Taken Hydras can summon Blight Bombs, as well as Teleport, Summon Oracles and Detain Guardians, or summon a circular shield like the Templar, protecting it from all forms of damage, that can only be broken by the Aegis.
The Taken Hydra returns in Joker's Wild, and appears exclusively as a boss in the first room of The Reckoning. It possesses two shields instead of the usual one, which rotate at a faster rate than normal Hydras. It can also rapidly fire off Darkness bolts, which blind any Guardians hit by them.
Known Taken Hydras