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.
They have not made the final cut in any of the games. However, it may have inspired a Flood form from Halo 3 known as a Flood Tank Form. The appearance is similar but the Tank has a hollowed stomach and is a darker color. They perform the same job as they did in Marathon, slow, strong moving creatures. They even have a similar growl which indeed sounds like "drinniol". The tank form is even stronger as it can take a player out in one swing on Heroic. But they aren't nearly as resilient, and are vulnerable from Energy weapons and fire.
Marathon Connection
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.