Have you ever heard of a buzzer that neighs? Now that is a real attention-getter! Barnyard Answers Buzzers are available from Discount Office Items and, while intended for children, I can think of a few (okay, a lot) of adults who would get a real kick out of these.
You can create unique photo books, cards and posters using your own photos on Shutterfly. If you have a child (of any age!) involved in horses, gather a bunch of photos of him or her with horses and ponies and mark their progression as they advance in the sport by putting the pictures into a photo book. That would be a treasure to keep forever, and a special gift they won't get from anyone else.
How about a clock? I listed a link previously to CafePress, and clocks are available there with a variety of equine designs.
Horse picture frames are great, too. I found one from England that I especially like from A1 Gifts, but a little hunting will find many more. Another English company, Animal Den, has tons of different horse items from horsy socks to throws and pillows to licence plate holders.
I also found some really different horse-themed items at Collections Etc., including wind chimes, accent rugs, even a chair cover. I especially liked the buckskin horse throw blanket and wouldn't object in the least if someone gave me one!