How to I set a prefab spawning in groups to have different textures

Hello I am developing an alien game but have hit a road block. I have two aliens that spawn in groups and are together when they spawn. They have the same textures every time they spawn. I have created more textures but do not know how to make them change as they spawn. If it is possible to make the same prefab spawn in with different textures could somebody show me how or tell me.

Thanks!