Robot Hat

This drops off two water enemies, so farming it means deliberately picking fights you would normally swim past. The rate is low enough that hunting it on purpose is a poor use of an afternoon. Most people end up with one by accident during a fishing trip or a swim through a flooded cave, and that is the sensible way to get it.

The hat is a nod to Cave Story. Quote, that game's silent robot protagonist, wears the same headgear, and the likeness is clearer in the official artwork than in either game's sprite work. The choice of piranha as a source looks deliberate too, since Cave Story has a piranha shaped boss called Ironhead.