For some, having a dog is not an option because of a fear, allergies, or even because they are considered unclean. But that doesn't mean that they have to miss out on the independence and comfort that a service animal provides.
The ADA revised the provisions on service animals to include the use of miniature horses. This is a victory for people that can't use dogs for whatever reason.
Horses are good choice for a service animal for several reasons. The average working life for a service dog is 6-8 years. Since horses in general have a longer life span than dogs, the same is true of their working lives, 25-30 years.
The sound of the horse's hooves also do a better job alerting someone of the changing ground cover than a dog's paws. Hooves echo differently on tile, wood, and carpet.
Another plus to miniature horses is that they don't get fleas.
I'm not suggesting that miniature horses are overall superior to dogs as service animals.
The horses require enough space for outdoor living when they aren't working. (Otherwise they can get health problems from being inside all the time.)
They also don't look for companionship in the same way that dogs do normally. And they aren't as playful as dogs.
Dogs have been trained to be service animals for people who are blind, deaf, diabetic, prone to seizures and the list goes on. Currently miniature horses have only been trained as guide animals for people who are blind.
I was able to get a lot of information from The Guide Horse Foundation who actually supply guide horses free of charge after the person complete the training course. Unfortunately, they aren't taking any new applications right now because they are overwhelmed with requests.
But you can watch this video of Panda a miniature horse that was clicker trained as a guide horse and see how this works.