The type of terrier also comes into play, for example most Bedlington/ Scottish terriers are so far removed from the original working stock they have lost the true hunter instinct. Whereas the Lakeland, Border, Plummer type are more closely related to the working strain. They WILL hunt which meansd rabbit chasing and digging.
If this dog is to be used with your mtb do you think you should get a more “leggy” dog!
Perhaps the ideal MTB dog is a Trailhound, manmade scene following foxhound- now they came cover some distance