Mountain Dog is a type label rather than a single breed. It covers big farm and guardian dogs from Europe's mountain regions, including the four Swiss breeds such as the Bernese and Greater Swiss Mountain Dog, and livestock guardians such as the Great Pyrenees, also called the Pyrenean Mountain Dog. Shelters often use the name for mixes of these breeds, so adopters may meet anything from a 50-pound tricolor farm dog to a fluffy white giant.
What these dogs usually share is heavy bone, a thick double coat and a history of hauling carts, driving cattle or guarding flocks. Many are calm, devoted family dogs, while guardian types tend to be independent, watchful and prone to barking at night. Expect lots of shed hair, sensitivity to heat, and the training and joint care that any large, strong dog needs.