Forerunner fauna and flora
From Halopedia, the Halo wiki
Halo Dinosaurs are native lifeforms to Halo, possibly only on Installation 04. They are both bipedal and quadreped in stature, and somewhat resemble Earth's dinosaurs.Cite error: Closing </ref> missing for <ref> tag
Although they did not make it to the final cut of the game, this does not exactly mean they aren't part of Halo as a whole. Engineers for example didn't make the final cut of Halo either, additionaly they were actually depicted in the Halo Novels. So who knows if these animals are meant to be there, and just unseen. They seem to have been created just to add some more depth to the Halo Mythos.
The Birds of Installation 05
On the levels, Delta Halo, Regret, Gravemind and High Charity, there are birds flying around in the sky. At first they seem like dinosaurs, but they are seen in High Charity, so it is also possible that the Covenant brought "food," and they are seen on 05 because they would breed quickly. But then, if they are dinosaurs, the Covenant would bring specimens to their holy city. Basically, it may never be known if they are dinosaurs, or food brought by the Covenant. Bungie may be, very well the only ones to ever know.
