M'galekgolo

Mgalekgolo, more commonly known to Humans as the "Covenant Hunters", serve as large offensive weapons against the Covenant's many enemies. They serve in pairs, and should one fall, the surviving one will go berserk.

They're actually made of many worm like creatures called Lekgolo

Origins Of the Hunters
The Mgalekgolo were brought into the covenent in the 23rd century through the event known as the "taming of the hunters". Their homeplanet, Te, was a gaseous planet with an atmosphere composing of mostly methane and phosphorus. The planet wasn't always gaseous: the gas only preduced by the Mgalekgolo colonies eating through forerunner artifacts:this was seen as a great herecy by the covenent which caused them to try and destroy all Mgalkegolo. The Mgalkegolo, however, won most ground battles and destroyed massive armies of covenent and even managed to kill the Arbiter which had been called to the battlefield. However, the covenent still managed to get the upper hand of the war: the Mgalekgolo might have been strong on the ground, but they had no defense against the glassing procegeres the Covenent eventully took. The Mgalekgolo, to save themselves from extinction, allowed themsleves to be abosorbed into the covenent: their armour and weapons provide by elites. Because of this event(and many others), the Mgalekgolo have a massive respects towards the elites.