From Destinypedia, the Destiny wiki
Hydras are hulking, segmented machines, described as resembling "robotic centipedes," which hover above the ground possess formidable defensive and offensive capabilities. Hydras utilize unidirectional defensive circular shields and  are equipped with a powerful weapon called the Aeon Maul. In addition to being formidable combatants, Hydras serve as major processing nodes for the Vex network. Like many Vex units, they vary in size, with the largest (such as Argos, Planetary Core) easily measuring hundreds of meters across.
Hydras should be engaged with extreme caution. Not only are their circular shields impenetrable and invulnerable to attack, but they are also armed with Aeon Maul, a type of Vex heavy weaponry. These explosive, Void damage-dealing weapons are fired in rapid succession and are capable of quickly blasting away at a Guardian's shield. They will continuously bombard the player's position on the battlefield from long range and will suppress them if left alone. Even after they are killed, Hydras should still be kept at a distance, as their head will explode upon death. This explosion causes massive damage to, and can even kill, Guardians.
Hydras now have three semicircular shields as opposed to just one, giving them stronger defenses, and their death blasts have a larger radius.
List of appearances