So someone said that Doc is an Energy Elemental and the Goon is a Dark Energy Elemental. Actually, that is not true. In Slugterra S4 E10 which is The Fall of the Eastern Champion, it shows that when the Emperor goes out of the Deep Caverns, he found a guardian slug (which is a Boon Doc), then he turned it into a ghoul, which was the Goon who took Eli's and Junji's minds. In the Return of the Elementals, it showed that the Goon cannot be healed, but he was not born as a ghoul. Maybe because of the Emperor's massive dark power, he turned the Guardian Boon Doc into an uncurable ghoul.
Those were only theories and have been out-dated for a while now (from 2014) and "The Fall of the Eastern Champion" didn't air until a few months ago, so until then the Goon's origins were unknown. But now that we know that he was in fact a normal slug and a Guardian, it does explain a lot about him and his abilities, although they also raise some more questions as well.
However on a side note, the "Dark Elemental" theory might very well be factual. After all there must a least 1 slug or ghoul that's the forbearer of dark energy in slugs/ghouls.