Unidentifiable Synodontis

I have recently aquired a 3 inch "Synodontis Nigriventris" (Upside Down
Catfish). Although this fish does swim entirely upside down, it to does
not exhibit many of the other identification marks of a true
Nigriventris. The fish is entirely black with long wiskers and
resembles a Arius Seemanni much more than a Nigriventris. It is also
extremely aggressive and predatory. It consumed 24 guppies, in one
evening, it's first night in my communal tank. It has since been
isolated in a small tank until I can figure out what it really is.
Does anyone have any information regarding a fish that matches this
description? Thank you in advance for any information.