I presume it exists, but are there any documented cases?
My understanding is that animals with brain abnormalities usually die. Though I think one of my cats is autistic, but I’ll never be able to prove it. 
I swear one of my mom’s dogs, a Shepherd/Husky mix, is retarded. An unbelievably beautiful, elegant-looking, huge dog, but mentally subnormal. Runs around in circles snorting and coughing.
For a short while, I raised persian kittens. My first litter, consisted of one kitten. It delivered breach, and the placenta seperated before it’s head delivered. It was without oxygen for 45 seconds to a minute. When I finally got it out it wasn’t breathing, so I did some kitten rescue breathing. She survived, but she was strange. She would stand in the middle of a room, looking around like she’d forgotten where she was. She didn’t play , even when she was a kitten. I sold her to sisters in their 70s, so she didn’t have to be too smart. They loved her. She was a true lap cat.
I’ve trained a lot of dogs, but have only ever encountered one that was untrainable. A $3000 blue merle collie with a “high white factor.” Dumb as a sack of hair. Only dog I ever threw in the towel on.
A few years ago, I read about a troop of monkeys where one member was mentally deficient but was able to survive and reproduce. This animal was supported by members of the social group. Also, if memory serves, they were “urban monkeys” so food was not a big issue for them.
I will try to find the source of this. For now I would say that a mentally handicapped animal could survive if it was part of a tightly-knit social group and were food is plentiful.
My grandma’s dog was like that-- completely untrainable in anything but his eliminatory functions.
He really was a great dog-- gentle natured, happy and loving. He was just stupid. If a toy rolled beneath a chair, he would stand and stare at it, and eventually begin to bark at it. He never figured out that he could scrape it out with one of his paws, even when I tried to show him.
Amazingly enough, he turned out to be a seizure dog. My grandma gave him to an eldery friend because she could no longer keep him. The old man came to my grandmother with tears in his eyes to thank her one day, claiming that the dog had saved his life. The dog would bark to warn him whenever the man was about to go into a seizure.
Our farm once produced a horse (accidentally) who was the result of very close inbreeding, and from an undersized (twin) mare.
He was a very friendly animal, with a nice disposition. But when he was old enough to start training, he turned out to be very dumb. He would literally forget things that he had been doing well just the day before. For a while, we thought he was being very resistant (unusual, given his disposition) until we realized he just didn’t remember from yesterday. So it took a long time to train him.
We eventually sold him to a retired postmaster in a small town, who wanted a horse to drive around with his many grandchildren in the cart. Very good match, because nothing seemed to upset this horse. He called us about something that happened: on the 4th of July, while some of his grandchildren were in the cart driving this horse, some kids threw a lit string of firecrackers underneath the horse. But all he did as they exploded was stop, stand there, and stick his head down between his front legs to see what was happening underneath him. The owner was very happy, said that was exactly the kind of horse he wanted for his grandkids.
So this horse ended up with a happy home. But I’d say he was close to being retarded, if you could measure that in horses.
I suppose it depends on how you define mental retardation.
Animals - certainly dogs - can have anxiety disorders, senile dementia, obsessive/compulsive disorders, can get depressed. They can be socially dusfunctional. Probably much of this is more due to early socialization and learning than inherently poor hardwiring. Though dogs (the animal I’m most familiar with) are born hardwired with varying levels of mental stability and soundness.
There’s a branch of veterinary medicine that deals with such issues - veterinary behaviourists and neurologists. There are medications and treatment protocols for many issues like these, quite well documented I imagine, though I don’t have cites on hand.
In a lifetime of owning dogs, I’ve only had one who I would call retarded. I can document it right here, she’s lying next to me on the floor! 
She was a ghetto shelter Rottweiler and is quite mentally unsound. Anxious about life in general, nervous, high strung, has real difficulty learning new things and is prone to obsessive behaviours. I’ve had her four years and despite extensive training, medication, herbs and just about everything else, she’s just not right.
Sweet, no aggression issues, so she’s still here. But mentally, she is pretty retarded no matter how you define it.
Are you sure it wasn’t just deaf? Merles can go deaf if they have too much white. Something about the hairs in your ear needing pigment to develop.
Nope, not deaf, just stupid. He knew his few handful of commands, and would do them listlessly, but he was a brick wall for learning anything new.
I recently adopted two adult miniature australian shepherds. One is as smart as a whip–I’m amazed at his word recognition, and how quickly he picks up new words. Kenji and I can make up a game in one play session, and he will remember it the next time. He knows that “kitties” refers of all of our cats, but “Jasper” is the tuxedo cat…he can form mental subsets!
Then there’s Phoebe. I think she has finally figured out the meaning of “sit”, but we haven’t really been able to do much more. Her movements tend to be a little bit jerky, and her eyes don’t have as much of an “aware expression” as Kenji. While it may take many tries, she can solve some problems…for example, climbing the stairs. On the other hand, I wonder if she’ll ever be able to figure out what doors do.
I have the good fortune of working with several veterinarians, one of whom is a neurologist. He did a brief evaluation and believes that Phoebe does indeed seem to have underdevelopment of the cerebellum, thus causing the jerky movements. It does not appear that she has suffered any head trauma.
Fortunately, Phoebe has Kenji, and wants to emulate him. They get along very well–when on a leash walk, she tends to sidle up and practically lean on him as they saunter along. Phoebe is a very happy dog and is always the most vocal with the celebrations every time I come home from work. With such complimentary personalities, they make an adorable pair.
On hot summer days my dog will go outside and sit in direct sunlight until she begins to pant and then comes back inside for the a/c. She does this in the winter or on cool days for the obvious warming effect but she cannot connect the dots to avoid the sun on hot days.
What’s worse, even though I’ve explained to her (with the aid of Excel spreadsheets and Power Point presentations outlining thermal and radiant energy and heat sources) that this behavior is counter intuitive, she simply licks my face.
Would this qualify her (or me) as being retarded?
I was once told that polydactyl cats have some syndrome akin to Down’s, but I am unable to confirm this with a cursory Google search. The closest I can come is someone saying they have also heard that, but it isn’t true and is based on the fact the polydactyl cats take longer to learn to walk/jump well because of their odd feet.
We used to have a polydactyl cat, and she was quite stupid. She was smart enough to learn that the sound of cellophane probably meant that a bag of Doritos was being opened, however. That cat would eat popcorn, bread, etc., and she LOVED Doritos. She got quite fat. I used to joke that she didn’t have cat Down’s, she had cat Prader-Willi Syndrome.
Sorry, not much of a GQ answer.
I have a 46lbs Jack Russell that likes to watch himself pee on his own foot.
Another anecdote, this time from the wild: I once noticed a male cardinal with a “broken” song (cadence twice as fast as it should be, and other abnormalities). The other male cardinals would pester him unmercifully.
I had two cats from the same litter: one was terrifyingly smart and “challenging” (she wanted to be the alpha in the house), and the other one could barely find her food bowl and never really figured out the litter box. There is no way she could have survived in the wild.
I think Smart Cat got a brain and a half and the other one got short-changed. Or Smart Cat tried to strangle the other one in the womb and cut off her oxygen.
I’m no expert, but that doesn’t sound like retardation. But it does sound like an illness of some sort. Hopefully his vet knows about it.
Sounds pretty retarded.
I heard a radio program once about animal husbandry and “heritage breeds”. The general thesis of the show was that by overbreeding for desirable characteristics, we have eliminated some basic survival character of many domestic animals in the last century.
Examples include the giant “Holstein” cows, with massive udders; such development came by narrowing the gene pool and severe inbreeding. SImilarly, large hogs that produce a lot of meat very quickly, but have to be restrained in cages because they are so dumb they will roll over on to their own litter. Sadly, for economic reasons these morons replace the capable animals; fortunately, we restrain them in cages or stick the food right in front of their face, so they don’t need significant survival skills.
A friend of mine swore his high-strung dalmation was visually appealling but dumber than dirt and hyperactive, while another said his german shephard was smart enough to follow simple conversation, like bringing slippers without being told because it was mentioned in the conversation with his wife a minute before.
But the short answer is - in the wild, anything retarded probably dies when weaned; since they will be either too dumb to hunt effectively, or too dumb to run away properly. There might be minor exceptions in group/herd animals where “follow the leader” is a good strategy, as long as the social dynamics are not too subtle. OTOH, mate selection generally is driven by gauging fitness of the other mate, so odds are any surviving animals would also have difficulty reproducing.
As for “downs syndrome”, IIRC that is a specific problem related to duplication of one particular gene in humans, so I have ot head if there is similar issues with genetics in other species.