You can see mentally challenged animals if you observe them carefully

Cognitive specialization makes cross-species comparisons more complex, while potentially identifying human cognitive uniqueness that is depicted below in a graphic form

Image source: http://Sciencedirect.com

Each animal has to be calibrated according to the species’ intelligence capabilities. You don’t go out and do that to all the animals to find out.

In neuroscience there are two primary ways to generate a genetic strain. One way is to knock a gene in or out. Using this method we can generate strains of mice which have similar symptomology and genetic alterations as Fragile X, Rett's Syndrome, Angleman syndrome, and Down's syndrome. (To name a few there are many many many others).

Do similar mutations occur in nature? Probably. Do the animals survive? Unlikely.

It is very difficult to survive in Nature with mental challenges. Animals are not human beings ( I know some human beings kill their own babies for various reasons, but this is rare). They don’t take care of mentally challenged fellow animals.

Animals kill their own babies, a behaviour called infanticide, for various reasons, primarily related to resource availability, reproductive strategy, and the survival of the fittest. Mothers may kill weak or sick offspring to conserve resources or ensure healthy babies get adequate nutrition.

The practice has been observed in many species throughout the animal kingdom, especially primates (primate infanticide) but including microscopic rotifers, insects, fish, amphibians, birds and mammals (1).

Any animal given a traumatic brain injury, developmental toxin, neurotoxin, high fever (for epilepsy), or anoxia (a condition in which too little oxygen reaches the tissues ) will develop brain damage just like a human under those conditions would.

Infection can cause mental disorientation in animals primarily by disrupting the central nervous system, particularly the brain, through mechanisms like inflammation, nerve damage, and the action of pathogens. This can lead to neurological signs, such as loss of coordination, confusion, and altered behaviour. You must have heard about rabies.

Examples of Infections Leading to Disorientation:

Rabies: Rabies, a viral infection of the brain, can cause confusion, restlessness, and ultimately, paralysis.

Listeriosis: This bacterial infection can cause depression, loss of appetite, lack of coordination, and circling in animals.

Chronic Wasting Disease (CWD): CWD, a prion disease in deer, elk, and moose, causes progressive brain damage and behavioral changes, including loss of coordination and unusual behavior.

Mad cow disease: formally known as Bovine Spongiform Encephalopathy (BSE), is a transmissible, progressive, and fatal neurological disease affecting cattle. It's caused by an abnormal prion protein that damages the brain and other parts of the nervous system.

Rift Valley Fever:This viral disease can cause severe neurological symptoms in animals, including disorientation, confusion, and lethargy.

Even if some mentally challenged animals survive this, differentiating between an animal with a mental disability and a typical animal involves observing specific behavioural and cognitive changes.

While animals may not have intellectual disabilities in the same way humans do, they can experience cognitive impairments, anxiety, and other mental health issues like disorientation, difficulty remembering things (like where food bowls or doors are), and problems with problem-solving (2). These can manifest as unusual aggression, excessive barking or chewing, changes in sleep patterns, or even difficulty interacting with owners or other animals. Animals may exhibit signs of anxiety, such as pacing, hiding, or excessive grooming. Some animals may exhibit unusual or repetitive behaviours. An inability to learn new commands or tricks, even with consistent training, can be a sign of cognitive difficulties. Some animals with cognitive or mental health issues may show decreased interaction with their owners or other pets. In some cases, physical signs like head tilt, sideways walking/running, or loss of coordination can accompany cognitive problems.

All these have been observed in domesticated animals, especially pets. They have been observed in animals kept in zoos. Perhaps they occur in wild animals too.

Some breeds are more prone to certain mental health conditions than others. Older animals may also experience cognitive decline. Just like humans, animals have individual personalities and temperaments. What might be considered unusual for one animal could be normal for another. Stressful environments or a lack of mental stimulation can contribute to mental health problems in animals.

You have to observe the animals carefully to see all these things.

Now try to do that. You will definitely find animals with cognitive disabilities.

Footnotes:

1. Infanticide (zoology) - Wikipedia
2. Physical signs of canine cognitive dysfunction