Dog's legs. I take the point about what the majority use becoming the norm, but legs and tails do have functional differences, and these would require some differentiation in any useful language. So a "tail" would never become a "leg", not being capable of operating as one. Whatever you called the total set of extremities.

Languages do change, but rarely (I think) in nonsensical or anti-rational ways. Though the reason behind some of the changes may take a while to work out, and others may indeed be genuinely random.