Question: In the wild do animals with mental disabilities get cast out of the herd or group because they aren't very good at reproducing or surviving by themselves? Do they have mental disabilities?

Yes, this happens and this is natural selection at work. Those animals that are most attractive, resourceful and intelligent will be the first to mate, to have offspring and ensure that their genes are passed down the generations.

Still, there are many examples of species helping out sick or disabled members of their group. For example dolphins support sick or injured animals, swimming under them for hours at a time and pushing them to the surface so they can breathe.