Rat (species)

Rats are a species of rodent native to Earth, with the first "true" rats originating in Asia. They are distinguished from mice in their larger size and are often kept as pets or used in scientific study.

Appearances
They appear on most of the Campaign levels set on Earth, as well as the Multiplayer map Rat's Nest in Halo 3, though all the rats found on Rat's Nest are already dead, likely due to the fact that they would appear on radar and confuse the players. It is possible to kill them. When they are shot at or encountered they will squeal characteristically and will appear as small yellow dots on your motion tracker. When killed they will roll around in a prone form. Rats have been noticed around the butchered area of the barracks, most likely feeding on the killed marines.