Action Comics vol.1 #82 - Cover date March 1945
Superman comes very close to finally facing another supernatural foe but, in a twist reminsicent of Scooby-Doo, we find a man under a rubber mask.
A dam project is interrupted and undermined by the sinister, sometimes-rhyming Water Sprite, a horrific fanged creature who claims to speak for supernatural forces invested in the running waters. When Superman intervenes, he discovers that the costume and posture are all a ruse, and the Water Sprite is actually the man responsible for building the dam - he evidently wanted to destroy it in order to hide the use of shoddy materials, then blame the event on the inhuman Water Sprite.
Most of Superman’s foes at this point are the lightly comical, from Toyman to Prankster to Mxyztplk, so an apparently mythological monster-man makes for an interesting break to the rhythm.