Piece of Moon Squid

Master Mode is the only place this drops, so it is a trophy first and a pet second. The Moonling that trails after you is a shrunken copy of the head you spent the whole fight shooting at, floating along with the same single eye. It does nothing in combat and asks for nothing in return.

The tooltip calling it forbidden calamari lands on the running internet gag of labeling anything inedible that resembles food as forbidden something, and calamari is fried squid, which fits the boss it came off. The buff line points somewhere older. From Beyond is a Lovecraft story about a man who sees creatures living in a dimension folded over ours, and the Moon Lord is already a Lovecraft nod in the first place. The pet design also reads a lot like an Inkling from Splatoon.