Aliens spawn in the default Minecraft world but rarely. They are aggressive towards players & villagers to harvest for the next generation of Aliens. The more villagers the better. Do not approach an Alien of any kind unless you know what your doing! Or you will die!
However, In multiplayer they can be used as guard dogs &/or soldiers & can be used to attack other players. When near a village it should be fairly easy to breed Aliens so long as you have a live Alien or an Alien egg.