The rat is a large rat residing in the eco mine. They are used by Jak to turn large wheels which operate navigable platforms. They spawn from large pipe mechanisms in the eco mine's walls, where Jak must have them follow him to a wheel, quickly kicking them into position, where they will run in place endlessly.
Though their role is small, they can bite and will when they get the chance, so it is best to evade them as much as possible, at the same time encouraging them to go in the direction you want them to. They will only appear after all residing fodders have been destroyed.
Once kicked, it will flop over and look dead, though it will not disappear, and instead will get back up in a matter of a few seconds. Once one is kicked into position, the rest will flee away.