There are some fabrics out there with the ribbons that you win at horse shows. Makower did one as part of its Gymkhana line, and there is some currently available on eBay. The whole line is wonderful for little girls, but very hard to find.
Also, Springs Industries has the Shenandoah line of horse prints that is very pretty and definitely a little more on the feminine side. Jo-Anns has a version of this collection available at present if you can't find it elsewhere.
Other options that might be fun for a young girl are the Laurel Burch collections of horse fabrics, and there are some patterns available for those.