Ghoul

If the Ghost is the cute one, Ghoul is the nightmare version. Both belong to the "Spectral Tide" admin event and both appear under Foggy weather, but this one is tagged Limited and the wiki rates it extremely difficult: its movement is erratic too, and the -95% progress speed penalty is five points heavier than the Ghost's.

The game quote calls it a malevolent, ghost-like spirit that preys on the living, far more dangerous than an ordinary ghost. Its look backs that up. The wiki describes a translucent body rounded underneath and flat on top, with two black eyes that form pointed, evil-looking shapes, plus a dark, elongated figure. It is the opposite of the plump, light Ghost: longer, darker and sharper.

It shares two catch lines with that fish, "DON'T CURSE ME!" and "AHHH!", and a note on its page says not to mix the two names up. Bait, weight, XP and price are missing from the page, so the main facts to carry away are the Limited tag, the Spectral Tide, the fog, and the 95% slowdown that makes it the tougher catch.