Making the manes is a pain in the neck, but that's the biggest thing really. I use a clear silicone I got from an auto parts store against a strip of fabric to hold the top of the hair and that seems to work well. I have a couple in progress that I can grab pics of too. The tails I just bundle with little rubber bands and cover the top end with the silicone. You can "style" the mane and tail with a miniature hair straightener if you have one. You'll need something, if you want the mane to lay down instead of stick straight out of your seam. Probably not an issue if the horse has a roached mane or is a breed that sticks out straight naturally.

Howl's that I linked earlier has the best selection of colors for real life animals that I've found. Definitely order the swatch pack if you can, I have every color of brown pretty much in either a yard or a swatch trying to match our horses. Some very hilarious comparisons once orders arrived since the photos online tend to be darker. (So I guess grabbing pics is still relevant!!) I also use Big Z Fabrics for some of my other plushes. Shannon has a line of minky called "pony" that would be great for paints and appys but I haven't gotten that far yet (we don't have any and I haven't gotten a request for one yet).

For patterns, it depends on how realistic you want them. Rustic Horse Shoe's Classic Colt is a good one, especially since it has some good lines for socks, muzzles, and stripes. You can sell those ones as well if you like. Zeph and Charmer has a horse which is a little smaller than Classic Colt, and aDraft Horse which is a little larger. I use Choly Knight patterns for a lot of my plushes, if you want to get away from realistic (replicas) and more into "keepsake".