Natural animals going through the portal would stay the same. That horse (in my headcannon) wouldn't change at all. But if a magical animal goes through the portal, they'll turn into an animal of a simalar size, EX: Spike is roughly the same size of a small dog, so when he went through the portal, that's what he turned into. Also, the horse would be able to talk to the ponies, the horse would just be unusually tall compared to the ponies, since ponies are just smaller breeds of horses.

Although, I don't have any theories as to why ponies turn into humanoids, instead staying the same minus wings and horns.

Cool

Kawaii~! These are actually pretty cute looking!

Christmas isn't over though, it doesn't end til Janurary 6. :D