Rat's Nest has rats in it, but they're all dead... Last Resort and High Ground have seagulls Guardian and Epitaph have dinosaur-like birds The Pit has Swalllows flying around near the ceiling.
I think nearly all the maps have animated sky items. Standoff, The Pit, Last Resort have birds. Avalanche and Snowbound have snow. Narrows has a foggy mist. Rat's Nest has pelicans and hornets flying around. Highground's 'sky' items are the fish in the lake and Valhalla probably doesn't have anything because of the beams that get shot from the bases. So most of the time, the maps will either have environmental effects or animals. I think the only exception is Ghost Town (even Foundry has the dusty air).
I thought this thread was going to discuss the animals Bungie tried to incorporate into the original Halo: Combat Evolved. You could litterally ride on an animal like you could a mongoose. lolz
Valhalla - Butterflies The Pit - Birds Guardian - Pterodactyls High Ground - Birds (possibly pelicans) Longshore - Fish (possibly)
Yeah, but thats reeeaally old news. Like, Halo: The 3rd Person Shooter for Mac type old. That's about a decade ago now!