The Red Rex is a Dark Gaia monster that appears in Sonic Unleashed. Essentially, this creature is an enhanced version of the Little Rex, the main differences between the two variants being that the Red Rex has a red-hued body whereas the Little Rex is green.
Aside from a difference in colour, not much else sets the Red Rex apart from the Little Rex. These creatures attack in swarms and rely on their numbers as they are still not very strong, though they can last a little longer against Sonic the Werehog's attacks.