Ephemeris

An elusive being which dwells in dark places such as the Magical Temple. It is barely seen and quickly flees the site. Adds to the creepiness of the area.

Regardless, this creature is still killable but can only be killed while being barely visible at the screen's edge, so it doesn't vanish. Yes, if you're looking to get full progress, this creature is required, however they are rather common. The idea was inspired by assets in the temple in Environmental Station Alpha.