Slinger Thorn is Slinger Ammo Monster Hunter World (MHW).
Slinger Thorn ammo (Thorn Pods) sticks to the target and explodes when hit with an attack.
Thorn Pods can be used to stagger Monsters both when the Pod lands and when it is later struck by an attack. They also cause Stun buildup when attacked to the head.
Source: Dropped by various Monsters and Soldier Helmcrabs found in the Rotten Vale
See Slinger for more information about slinger ammo