Definitely a syno catfish. Probalby Synodontis nigrita (Lace catfish), possibly Synodontis eupterus. Both are fairly common in the hobby, and even found at megamarket chain stores.
Just to offer a theory on how it got there, I've heard that some syno's will deposit their eggs in with the eggs of mouth brooding cichlids. I've never seen it happen, but here is what I've heard: Basically they wait for mouthbrooders to start spawning, then sneak their eggs in the mix. The baby syno's will then eat the cichlid eggs until they are released by the mother cichlid. So basically a cichlid gives "birth" to a catfish.
So my theory, perhaps you bought a cichlid which was being kept in a pet store tank with a syno. At some point while in the pet store, the cichlid attempted to spawn and a syno snuck some eggs in there. Then when you brought the cichlid home, she eventually released a developed syno catfish into your tank.
