I too have wondered "why a horse as a service animal??"

From the New York Times magazine article "Creature Comforts":

"In fact, Edie says, there are many reasons: ...But the biggest reason is age: miniature horses can live and work for more than 30 years. In that time, a blind person typically goes through five to seven guide dogs. That can be draining both emotionally and economically, because each one can cost up to $60,000 to breed, train and place in a home.

“Panda is almost 8 years old,” her trainer, Alexandra Kurland, told me. “If Panda were a dog, Ann would be thinking about retiring her soon and starting over, but their relationship is just getting started. They’re still improving their communication and learning to read each other’s bodies. It’s the difference between dating for a few years and being married so long you can finish each other’s sentences.”

Edie has nothing against service dogs — she has had several. One worked beautifully. Two didn’t — they dragged her across lawns chasing cats and squirrels, even pulled her into the street chasing dogs in passing cars."

I love this image...

"There are no sidewalks in Edie’s neighborhood, so Panda led her along the street’s edge, maneuvering around drainage ditches, mailboxes and bags of raked leaves. At one point, Panda paused, waited for a car to pass, then veered into the road to avoid a group of children running toward them swinging glow sticks. She led Edie onto a lawn so she wouldn’t hit her head on the side mirror of a parked van, then to a traffic pole at a busy intersection, where she stopped and tapped her hoof. “Find the button,” Edie said. Panda raised her head inches from the pole so Edie could run her hand along Panda’s nose to find and press the “walk” signal button."

Let's hear it for service animals. There are a lot of wounded vets right now that are waiting for theirs. The dogs are also being trained by prison inmates in many states.