What is the official name of a border collie and mountain fiest mixed dog?

There is no official name for a Border Collie and Mountain Feist mix. Mixed breed dogs are often referred to by their parent breeds, such as "Border Collie/Mountain Feist mix" or "Border Collie-Feist mix."

You could also come up with a name based on the dog's personality or appearance, or simply choose a name you like!