From Halopedia, the Halo wiki
Butterflies are insects with large, often brightly colored wings capable of conspicuous, fluttering flight.
Butterflies first evolved on the planet, Earth, in the Sol system. In accordance with the Conservation Measure, the Librarian indexed multiple species of butterfly to ensure their survival during the Great Purification. They were preserved on Installation 00 and were reintroduced to Earth in the years following the firing of the Halo Array. However, a number of butterfly species remained on the Ark indefinitely and flourished there. When humanity rediscovered the Ark in the twenty-sixth century, the descendants of these butterflies were encountered. The butterflies of the Ark were incorporated by UNSC cartographers into a simulated War Games environment.
In Halo 3, butterflies can be found in certain areas of the multiplayer map, Valhalla. They are found in groups of three and remain in the same location. Groups of yellow butterflies can be found at the center hill (where the Spartan Laser spawns) and at the base of a tree in the middle of the map. They are unaffected by players' actions.
List of appearances