When a fox sees a player they trust being attacked by a wolf or another player, they will get angry, growl at, and have the ability to bite the wolf or player, but they will still run away. Also, they will only pursue players when they're sneaking. This could be a problem in PvP with foxes involved since they won't always attack other players if they aren't forced to be close enough using leads or corners.

To reproduce, summon a fox, spawn a baby fox inside it (make sure the baby fox isn't following the adult), spawn a wolf, attack the wolf, get bitten by the wolf, and the fox won't chase after the wolf to attack it. Instead it will pace around the wolf and growl. They will only pursue the wolf if it has invisibility.