I did some research for you and found an answer to this question from a veterinary neurologist at New Bolton center: Here is the answer:
***** *****, DVM, Dipl. ACVIM, equine neurologist New Bolton Center. Here is her reply from 2012:
Dementia has not been well-described in the horse. However, scientists have observed an age-related syndrome similar to dementia in elderly dogs and cats that likely occurs in other species as well. This problem, "cognitive dysfunction syndrome," is best described in dogs. Interestingly, the brains of dogs that have cognitive dysfunction show pathologic changes similar to those that occur in people with Alzheimer's disease. So, just because scientists have not formally recognized equine dementia in the horse does not mean it does not occur, and in all likelihood aged horses might suffer from some of the same age-related structural brain changes that occur in other species.
Of course, we must consider other possibilities for behavior changes in horses such as brain tumors or metabolic diseases. It would be prudent to have a veterinarian perform complete blood work, including a biochemistry panel, and a complete neurologic examination to ensure there are no signs of a disease process that could be affecting your horse's behavior.
Just as there is no known cure for Alzheimer's, there is no known cure for cognitive dysfunction syndrome, and the same is likely true for equine dementialike syndromes. Veterinarians have used some drugs to improve cognitive dysfunction in dogs, with reported improvement, but I am unaware of them being used in horses for this purpose and can't even speculate whether they would be effective.
I hope this helps you. This is the best possible answer as it comes directly from an equine neurologists comments.
If you are happy with my service please accept my answer. Sincerely, DRB