Mountain Dog is a type label, so the dogs you meet can look very different. Swiss-farm types such as Bernese and Greater Swiss are broad, heavy-boned and often tricolor. Guardian types such as the Great Pyrenees are leaner and usually white or cream. Many shelter dogs are mixes, so a listing's size, coat and ancestry tell you more than the label does.
These dogs were bred to haul carts, drive cattle and watch flocks, and many still like having a job. Most are loyal and calm with their own people, though guardian types can be independent and vocal. Plan on heavy shedding, regular brushing and a dog that sleeps wherever it's coolest. Training should start early, because a stubborn 100-pound dog is hard to redirect.