Player (myself) Cannot Damage This Enemy

While on the hunt for a Humbaba, I ran into this fabled creature that I was NOT able to damage. Fireballs did not work, crushing, chopping, piercing, spells, and bullets/arrows from myself did nothing to it. Sensing I could do nothing to it the spider also ignored me completely, neither aggro'ing from the NPC or Player. My trusty Recruit was able to do damage to it however. Eventually they killed it and I was able to skin it. If it did aggro at all, it would have been an unfairly one sided fight.