Personally I'd get an ecogold saddle pad because they're so amazing!!! Or the half pads if you know they use half pads! If you know their horse's blanket size you could also get a back on track mesh sheet! They're amazing!! You could also maybe get them a customized leather halter with the name plate (again if you know their horse's size). Good luck!!
Horse gifts are the best gifts (am I right?!). But sometimes I know it's hard to pick out the one so here are my favorites ranked from $ (inexpensive) to $$$ expensive.
1: Engraved nameplate bracelets. $
2: Charles Owen helmets $$$
3: Embroidered Saddle pad $
4: Embrodered Jackets $$
5: C4 belts $ (I even have a 10% of code: C4GU7AQ)
There you go!! Enjoy :) all of the top 4 can be found on SmartPak, and the c4 belts can be found on c4belts.com