It probably won't win, but I'd expect the miniature-horse service-animal community to put forth some lawsuits, probably finding an unscrupulous lawyer that doesn't understand 14 CFR. They are service-animals, but the DOTs reasoning seems pretty solid, they can't sit in a lap and be secured like a lap-child nor can they fit under the seats, therefore, they cannot be secured, which is a requirement for all cargo, which everything other than people in the cabin is.