The Monaxi were a "trader" species which sold "lesser" species as slaves to "more developed" civilisations. (COMIC: Medicine Man)
The Monaxi worked with Mister Ebonite to capture individuals and put them in the Arena of Fear. (COMIC: Arena of Fear)
Biology
The Monaxi were a composite of material and energy. They looked like ornate flying saucers. But this was just a protective shell.