I have seen a print that is all horses, in shades of brown and tan, something like that is neutral enough that you could combine it with brighter colors, and it would be make a girly horse quilt. I've also seen in an old issue of Quiltmaker (really old, like from 10 or 15 years ago, but if you happen to have back issues, you could check through them) a block that was a horse. Don't remember if it was paper pieced or what. Anyway, if you do a horse block, you could make the horses any color you wanted, from hot pink to more natural colors.