is
If this fox is currently defending a player it trusts.
Foxes will naturally attack zombies, drowned, husks, zombified piglins, skeletons, wither skeletons, strays, phantoms, silverfish, endermites, spiders, cave spiders, vexes, vindicators, evokers, pillagers, ravagers, blazes, endermen, goats, and pandas when these mobs attack the player.
They will only defend the player if the player was attacked though, they will not fight for the player if they attack, unlike tamed wolves.