Relentless immortal monkey

In Matlan Waterways I've been chased by an immortal small monkey or ape who chases me relentlessly, cannot be hit even by area damage but can attack and damage me preventing me from using portals.

Not a big deal, but might be worth looking into what makes mobs immortal in case it could happen to a soul stealer hasted rare monster.

Thanks in advance.

The same issue has just happened to me. One of the feral primates is chasing me and it will not die no matter what I do. The monkey cannot be targetted by attacks. Cloud arrow poison does not affect it, even with Pathfinder's Contagious Contamination. When placing the mouse over the monkey, the monster tooltip does not appear.