He's a Makuta with a disfigured corporal body. I think it said somewhere in the lore that his current form was a result of exposure to the swamp mutagen in Karda Nui. He probably looked a lot different prior to the mutation, possibly with a more humanoid appearance and bat like features like Antroz or Mutran. Or he could've looked like Vamprah, which would basically make him look more like how he does right now, the only difference being that he would look more akin to a bat than an insect.