I'm a hunter seat horsey girl, so in addition to the great ideas you've already been given, I always appreciate:
1. Horse treats, packaged in a cute/creative way (critter-proof containers are always nice). Bob's peppermints or starlight mints, carrots, iced oatmeal cookies, and apples have all met with the horsey seal of approval. There are some DIY horse treat cookie recipes out there too - would be cute with a personalized "Pony Treats" cookie jar or something like that.
2. My hands are always cold, so fingerless gloves or mitts for grooming/tacking in warm style: also ski-style earwarmer band things are great, and cowls (instead of scarves with loose ends that are a safety hazard) are really nice, too. Warm socks that fit slimly underneath riding boots and breeches (lightweight fleece is good). Fleece zip-up vests and pullovers are nice for riding in cold weather, too.
3. Lip balm, moisturizer, hand sanitizer, break-resistant water bottle for post-riding hydration, maybe packaged in a small insulated bag made from equine print fabric? (I carry these with me everywhere, I also ride at a barn about 40 minutes from my house.) Maybe something like this for the bag: http://planetgreen.discovery.com/home-garden/lunch-diy-accessories.html
4. Anything personalized with her horse's name - like a hanging stall sign, baseball cap to cover up helmet hair, keepsake box, photo album or frame, see also #5 below.
5. Home-etched glassware with horse-y silhouettes. I haven't tried this yet, but think it would be pretty easy to make a stencil from clipart, and then use either a glass etching kit or frosted glass spray paint (you're only supposed to use the paint for decorations, so this wouldn't work for anything that would contain food). Or personalize some mugs or wineglasses with food safe glaze.
6. Have a coffee mug, t-shirt, cap, or tote printed with her horse's photo on it.
7. If you like woodworking, a nice grooming box, tote, bridle rack, saddle stand, or anything like that would be great.