One of my weekly newspapers had an article about a woman in Florida who has a miniature horse (not a pony) for a seeing eye service animal.
It stays in the house and nuzzles her hand when it needs to go outside. In restaurants, it patiently stands there while they eat.
The advantage of horses over dogs for this is that they can live 20 years, so you wouldn't have to be getting and training new ones as often. They tried to get the organization for the blind to certify horses as acceptable service animals, but it didn't happen because the organization didn't want to encourage the use of all kinds of animals as service animals.
She did talk an airline into letting the horse fly in the cabin with her - it stood next to the bulkhead the whole trip. I can imagine it would take some fast talking to convince an airline to allow this, just because of what comes to mind when a person says the word 'horse.' This particular horse stands 27 inches tall and weighs about 170 pounds, so isn't much bigger than large dogs, but horses don't fold up as easy as dogs. I guess you could get used to it.
The biggest challenge in the training was to teach the horse not to 'freak out' - her words. I don't know a lot about horses, but know that they can be stubborn and refuse to go where you want them to, such as going through water or a gate. But I guess if the woman can't see where they are going, wherever the horse leads her would be fine with her, as long as she ended up at her final destination.
According to the article, there is one other seeing eye horse and a seeing eye goat out there somewhere. And no, the society for the blind wouldn't certify goats, either. I have a harder time conceptualizing a goat as a service animal because they'll eat anything - just snatch a sack of whatever out of your hand and swallow it whole. But I shouldn't discriminate, maybe they are really suited to it.