Growin' up in farm country, I saw a lot of cows, cats and dogs without tails. Many times it was a injury when very young. Many times it was a hairball mixed with poop that stuck too long and cut off blood long enough for the limb to die. I could see an adult loosing it in a fence, a fight or even being shot off by a hunter that stop swinging when shooting at it running. I shot coyotes with 3 legs that were very healthy and fat. Doubt if loosing a tail would hurt a fox much unless in a very cold climate. Any 'yote with mange bad enough to loose their tail, would probably loose their life.