I definitely remember Rex. Appeared in the first two Mario & Luigi games and Super Princess Peach as well as Super Mario World.
Either way, he'll probably make a comeback soon. Nintendo always brings back enemies from classic Mario games at some point or another; just look at the Hammer Bros, Magikoopas, Chargin' Chucks, Koopalings, Boom Boom, Reznor, etc. If there's ever a New Super Mario Bros 5, I'd bet anything Rex will appear it.
For gaming news and Wario discussions, check out Gaming Reinvented
and Wario Forums
As for Mario's Nightmare Quest? Well, it's currently on Fusion Gameworks, ROM Hacking.net or the GCN at the moment.