As you can see in the screen capture to the left, the Miniature Horse is a limited edition animal that can only be purchased from the Market and it costs 26 FV Cash. If you want it, you should hurry since the offer is only available for 10 days!
The good news is that the Miniature Horse is a breed-able animal, meaning that you can place it in your Stall and have it produce Miniature Foals as well! According to Zynga, you need two Mini Horses to breed a foal but some players have already reported that they only got one and breeded a Miniature Foal. So it might be a glitch in the system that you might wish to take advantage of while it lasts!
Did you purchase a Miniature Horse already?