There is actually a small amount of wildlife on Alpha Halo (Installation 04) in Halo: Combat Evolved:
- Moths are found in the swamps on the level 343 Guilty Spark
- Small worms are found in the sea on the level Silent Cartographer (only in the original game, not in Halo: Combat Evolved Anniversary)
In addition, sounds from other creatures can be heard throughout the game, such as owl-like bird calls and cricket-like insect sounds.
However, according to an old news story on Bungie.net, One Million Years B.X. (Before Xbox), now only found in the Wayback Machine, more prominent wildlife was also originally planned, but scrapped mostly for technical reasons:
The Fauna of Halo
The main reasons that the Fauna, a collection of pseudo-dinosaurs and herds of vaguely mammalian varmints) were dropped from Halo (and relatively far into the process) were the usual combination of time constraints, design issues and performance problems. But another, perhaps more fundamental philosophical reason was that the inclusion of another set of inhabitants of Halo would have detracted from the surprise, drama and impact of the Flood.
Problems with AI for the creatures, problems with how they would react and behave during action sequences and even things like basic herd mentality meant that more important features (like vehicles and finishing the game on time) had to take precedence.
Although a populated Halo ring is unarguably cool, the sense of abandonment and mystery is actually heightened without them. The deserted landscapes and eerily empty structures actually serve to enhance the mystery of the Forerunners. But that's an accident of design rather than a purpose.
The sad thing is that the assets created for the animals, the animation, models and textures, were all largely complete. But those dinosaurs were destined never to roam the ring. Unless Bungie one day decided to make a dinosaur racing game!