That horse is VERY patient. It looks very stressed out (ears back is a tense horse if no one is riding it. Otherwise they might be listening to the rider)

That said, it’s clear that the horse could get away or lash out but is choosing to tolerate this weird animal, even lowering its head to balance the dog. My guess is a combination of good training and good friendship.

Just enough brain cells to put up with a cute animal, just like me