There is a bipedal form of the Dragons known as Alcoons. These could be everything from simply an advanced form of their class of species in comparison to the Dragons themselves to genetically modified Space Pirate bio-weapons. At the same time, however, they could just as easily be the same species, exhibiting either extreme sexual dimorphism or even simply a metamorphosis within their species (with the Dragons likely being the larvae of the Alcoons in this latter case).