Miniature Horse Size?

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My granddaughter has a new miniature horse that she just got. He is a stallion. He is five months old and right now he is about as tall as My Newfoundland dog...What is the top size that he can attain to be called a miniature horse and to fit into a class for a show pulling a carriage. (this is what my granddaughter wants to do with him) Does anyone know? Also I would like to make a gift for her at Christmas and am wanting to make a horse blanket. Does anyone have a pattern. Could I use a dog coat pattern and alter it a bit?