Mr. Onus was a crew member of the ship RLS Legacy in its voyage to search for the legendary Treasure Planet. While acting as the ship's lookout, he was also secretly a member of Long John Silver's pirate crew and participated on the mutiny after arriving on the mystical planet.
Onus belongs to an unknown alien species, similar to a slug with stubby arms, a wide mouth and three pairs of long retractable eyestalks with a wide range of movement; making him perfect for the position of lookout.
After the battle for Flint's treasure and the activation of the ancient trap which causes the entire planet to explode; Onus is finally arrested along with some of his fellow pirates and presumably brought to trial.