I thought that they were just a plot device for Wiley Miller's comic strip Non Sequitur, but it seems that guide horses for the blind do exist.
Pigmy ponies can be smaller than the large seeing eye dogs (they weight about 25 kilos and are 50 cm tall at the shoulder). The ponies have good memory, an excellent night vision and are calm even in the noisy city environment.
Some blind people like them better than dogs because they live up to 35 years, while a guide dog must be retired after ten years - forcing the owner to adapt to a new dog. I also imagine that a guide dog would be a big problem for a blind person allergic to dog hair...
A pony costs about 4000 dollars, plus care and training, but they are given away for free; the trainers rely on donations.
Some (cute) photos can be found at the site www.guidehorse.com; they include a rather curious pic of a pony flying in the passenger cabin of a plane - it seems that they enjoy the same rights as seeing eye dogs.