Some of the better horses I've ridden in the mountains were work horse crosses although I don't know what breeds any more. I'm 6'4" and 230 pounds and those horses worked out very well for me in the high country. But most of the time a good, big Quarter horse or standard bred did just as well. I like well trained mules but they are expensive and a lot of them can be difficult to handle. Same thing goes for a lot of the mustangs if they were broken late in life.
And as one other guy mentioned a lot of ranchers have bred up some mountain horse lines over the years that can be pretty impressive. 1/4 or 1/8 work horse blood can give you an animal that is big, tough and still moves well and is a nice ride. He may not be papered but still a fine mountain animal.