The graphics files for this character are named "namazu*" in the Touhou Hisoutensoku application directory, so this is assumed to be his species.
Perfect Memento in Strict Sense mentions that carp-like monster fish up to ten fathoms in length have been seen in the Misty Lake where the Scarlet Devil Mansion is located. This may be the kind of fish Sakuya caught at the end of SWR.
Some of the attacks of the Unnamed Giant Catfish are a reference to the attacks of Fatty Whale from Kirby Super Star and Kirby Super Star Ultra.
The Giant Catfish is the only character who appears as a boss in a fighting spin-off, but is not a playable character.