They can't harm the rabbids,but they are watching them and can be hitted,more and more UFO's start apearing later.
In the episode The Rabbid Who Fell To Earth. John Charles investigates an area after a claimed UFO sighting. Cody mistakes a rabbid that is dressed up as a chicken for an alien. At the end of the episode an actual alien is tryng to communicate with Cody but in respond Cody kicks it (believing it is a trick) and sends the alien flying and after finally landing.The UFO picks it up.
The alien apears again in Raving Alien. Where he's UFO runs out of milk for it to fly. He asks the rabbids for help, who don't understand what he needs and keep bringing him wrong stuff. He takes a baby's milk bottle, but it's parents beat him up and drive away. And the milk bottle spills. However he sees a cow on Wesley's truck.So he crashes it and takes the cow's milk. He waves the rabbids goodbye and flies away in his UFO.