Adorable horse hoods... Can my horse squeeze into this one?

You would likely want to get a large. I purchased a medium slicker for my 15.2 Arab and it was far too tight and constricting (plain old navy blue just for the record lol!), and he is on the narrow side already. Though the material itself is quite stretchy, the openings and bindings will probably be too small. Imagine having to wear a too small sports bra all day--sure you can squeeze into it, but is it comfortable?