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, although it is not known for sure. 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.

From 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".