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Drinol concept art.
The Drinol are Covenant beasts that were kept contained, but did not make it into Halo 2 and it is unknown if they will be seen in future products of the Halo universe.
In the Behind the Scenes DVD, Drinol are described as huge beasts that are kept contained and segregated from the other forces. Possessing phenomenal strength, they are used to smash enemy vehicles.
Marathon Connection
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The Hulk greatly resembles the Drinol.
Bungie enjoys placing links to their previous game franchise Marathon into Halo.
From a description of an enemy in Marathon:
They appear in two levels in Marathon and like many of the 'conditioned ranks' are not seen again in Marathon 2 and Infinity. They are seen paired with P'fhor fighters and have no ranged attacks.