From Pathfinder Howl of the Wild, which isn't enabled.

Tail Guard

FEAT 8

Activity reaction

Prerequisites Thlipit Contestant Dedication; unarmed attack with your tail

Trigger A creature enters a square that would flank you.

Enemies who attempt to stab you in the back will be unhappy to find your lashing tail on guard. Make a melee Strike against the triggering creature. If your attack is a critical hit, the movement is disrupted. This Strike doesn't count toward your multiple attack penalty, and your multiple attack penalty doesn't apply to this Strike.

Pathfinder Howl of the WildORC