Well for starters, a real horse is always a surefire way to go. (especially for those without one) And if santa brings me one for christmas, I swear I would move to the north pole and become an elf for the rest of my life.
To really answer your question, I was thinking of getting some halters for some people (I'd rather not say who in case they come across this thread
) in coordinating colors with their coats (their horses, not their actual coats), and if it isn't too much more, getting the horses' names sewn onto the noesband.
Some horse statues are always good. Breyers are a bit on the cashy side, but they are really pretty and come in a variety of molds and colors. I would suggest looking on ebay for one, it could be a lot cheaper and there probably is a lot more variety online than there is on a store.
What about getting her a saddle pad? They make those in a lot of fancy patterns and styles. (If you ride western, of course)
If your mom likes to cook, Rod's Western Palace sells a bunch of cookware with horse stuff on it. Dishes, wine glasses, towels, curtains, you name it. They even had a calunder? (One of those things you use to drain pasta) with horse shaped holes in it.
Hope this helps!