Reports come in from time to time of tiny horses. For example, mini-horses of 16"-18" were reported in the Philippines in the 1970s. More commonly, however, such horses are reported as the steeds of fairies. Thus about 1920 a Mrs Hardy of New Zealand wrote to Sir Arthur Conan Doyle saying she had sone perceived eight or ten small persons riding tiny ponies. Lady Gregory, the Irish folklorist, in a book published in 1920, was told by a man from County Clare that he had seen a field full of men riding tiny ponies.
If such creatures exist and serve as the mounts of fairy folk, I suspect they are interdimensional creatures.