I would love mythological ponies;
-Sleipnir (black horse with eight legs)
-Unicorn (the traditional kind, with a goatee thingy and the look of a draught horse)
-Pegasus (winged horse, duh XD)
etc... There are more out there, but this is just a suggestion.
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My favourite idea
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Another idea is horses from literature and movies. This is a special idea I had.

These would be very special, so the adoption limit would be low, but you would have the same chance of getting each horse. The way they would be done would be kinda like this. I'll use Black Beauty for an example.
First you would draw/colour a horse based off Black Beauty, then, when the horse is ready, you would put in small scrolly letters the name of the horse, which is in this case Black Beauty. The name would be located either under the chickensmoothie.com writing, or on the left side of the horse. There are many horses from literature and movies, to name a few;
-- Mysty ("Misty of Chincoteague")
-- The Black Stallion ("The Black Stallion")
-- Sham ("King of the Wind")
-- Spirit ("Spirit: Stallion of the Cimarron")
-- Hidalgo ("Hidalgo")
This is just a small list, there are more out there!