The Ancient Shadow Booster was a warrior and Booster from 3000 years ago but unlike the Ancient Dragon Booster, the original Shadow Booster only gets a brief mention within the series and as a result, not much is known about him. In Episode 38, Artha Penn and Moordryd Paynn are shown a flashback from the past when their gauntlets mag together, of memories trapped within the draconium of their gauntlets, where the original Dragon and Shadow Boosters teamed up together to fight back an army of Orange Draconium Dragons. From this, it is suggested that the original Dragon and Shadow Boosters were enemies, just as the current Boosters are, but that from time to time they had to work together.
At some point, the original Boosters must have formed a strong alliance as the Shadow Booster was part of the The Main Draconium Colours of Balance needed to stop the Dragon-Human War. The ancient Shadow Booster was also the person responsible for trapping Armeggaddon, the leader of the Black Draconium Empire that the Shadow Booster was supposedly a part of, in The Shadow Track.
It is speculated that the original Shadow Booster was most likely part of the Black Draconium Empire, though it is never confirmed- however, their armour and empire symbols are the same. If they were indeed initially working together, at some point the Shadow Booster decided to break off and work towards restoring balance in the world and ending the war that Armeggaddon helped start. Supporting this idea is the character development that Moordryd Paynn, the current Shadow Booster, is undergoing- having started as an antagonist but then showing properties of a protagonist and breaking away from his father's plans for war , much like the original Shadow Booster may have done to Armeggaddon. This may be the reason for Armeggaddon being so interested in Moordryd and training him to be his student as the Shadow Booster, so that the past cannot repeat itself on him again.