Drinol

The Drinol are Covenant beasts that did not make it into Halo 2, Halo Combat Evolved, or Halo 3, 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.

Resemblances
Drinols were not included in any of the Halo games, yet are similar to other enemies.

Flood Tank Form
Drinols 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 tan color. They perform the same job as they did in Marathon: Strong, slow 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 a single swing from its mighty arms on Heroic and Legendary. They are nowhere near as resilient as their Drinol concepts, however, and are especially vulnerable to plasma and fire-based weapons. It is unknown if the Drinol was to have a similar weak point, or if it would just be a damage sponge, much like the Jiralhanae's first appearance in Halo 2.

Flood Juggernaut
They could have also inspired the Flood Juggernaut, which was in the game code for Halo 2 but was removed half-way through. Bungie removed the Juggernaut due to the fact that it could kill the Chief in one swipe.

It was possible that it could have been a replacement for the Hunters in the Covenant Loyalist military in Halo 3, but was instead scrapped for the Hunter to keep players from being confused about which alien was with which faction.


 * --Gamesradar article

Marathon Connection
Bungie enjoys placing links to their previous game franchise Marathon into Halo.

From a description of an enemy in Marathon:

They appeared in two levels in Marathon, and seem to be slow as mentioned above but also unintelligent. With spike-like organs on its skin. The hulk is pure muscle and most likely ate every hour or so.

Trivia

 * Drinols resemble the Hulk and they have a slight resemblance to Brutes.
 * It has been speculated that Drinols and Sharquoi are the same species, just a different rank, or a different name for the same species.