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 quadruped in stature, and somewhat resemble Earth's dinosaurs.[1] Although the larger animals' names are not known, the smaller one has been identified as a Blind Wolf.[2]
Although they did not make it to the final cut of the game, this does not exclude them from Halo as a whole. For example, Engineers didn't make the final cut for Halo either, but they were depicted in the Halo Novels. So who knows if these animals are meant to be there, but just unseen. They seem to have been created to add some more depth to the Halo Myths.
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 might bring specimens to their holy city. It may never be known if they are dinosaurs, or wildlife brought by the Covenant to use as hunting game. Bungie may very well be the only ones to ever know. It is most likely that it was life that developed on halo, because considering the fact that many Covenant believe earth traditions to be barbaric, hunting is probably seen in the same eye, also, the Covenant have probably developed more advanced ways to eat, the bringing of birds is very unlikely.
Also, since we know that halo contains a vast multitude of terrains and environments, much like earth, it is possible that life has been developing on halo over the 100000 years that they have been dormant, such animals like the dino birds. Another reason why animals may NOT be seen is because they have the calcium store necessary for the flood to infect, so the animals may have been destroyed by the guardians of the construct, and only avian-like animals survive since it was mentioned that Jackals do not have sufficient calcium mass to sustain the flood.
In Halo 2 these "birds" can be shot and killed though they are hard to hit and after being killed, they turn into a Splatter of red Blood.
