I had an Afghan hound that was dumb as a post.
An R-tard if there ever was one.
We had a cat we dubbed Woodpile Kitty because we found him stalking around on the woodpile.
He never interacted with the other cats.
He did not seek out humans for attention.
He did not hunt or chase birds or butterflies.
He did not relax when picked up.
We never heard him purr.
He sat for hours in winter in front of the hot air vent, then licked and scratched until his fur fell out.
We always referred to him as autistic. One day we came home and found him stiff and prone on the deck. He was about 4 or 5 at the time and he was just never quite right.