Sharquoi

In the Conversations from the Universe booklet found in Halo 2: Limited Collecter's Edition, an Elite refers to something called the Sharquoi. This has been assumed to be another Covenant race. In a deleted scene, a mural can be seen showcasing the Covenant races, including a very large creature believed by some to be a Sharquoi. However, this creature could also be a Drinol, which had not yet been cut at that stage of development. There is no direct evidence, then, to state that the Sharquoi is the large creature, although it is difficult to think what else it could be. Another theory states that 'Sharquoi' is the name of the Forerunner Ship in whice the Prophet of Truth embarks from High Charity at the end of Halo 2. Other theories put forward include that the Sharquoi is the Covenant name for the Scarab (unlikely, as no other Covenant vehicles have been given names) or even that Sharquoi are the antimatter bombs used to destroy the Athens and Malta orbital defense platforms.

From a Bungie Weekly What's Update Nov 19:

Q: What's a Sharquoi? A: Very freaking dangerous.

Frankie has talked about playing Halo 3, and fighting Grunts and "something larger than a Grunt," supporting the idea that the Sharquoi will be seen in Halo 3. KP also mentions fighting Grunts and a "CENSORED".

More evidence of the Sharquoi?
This painting done by the artist, Craig Mullins, under commercial work for Bungie for the game Marathon reveals a hulking figure, closely resembling the Sharquoi or Drinol.