An Addon is a term used for pokemon that participate in a total Pokemon competition where they joined the game after the game has already begun. This is not to be confused with returning pokemon, as addons join the game for the first time in the middle of the competition, although in come cases addons that get voted off return once again later on.
Examples of Addons
Eggs-some pokemon hatch from eggs in egg related challenges and end up joining the game.
Fossils-similar to eggs but are resurrected from fossils.
Pokeballs-in pokemon battle challenges sometimes teams are rewarded with one of the rental pokemon used
Newcomers-To keep things fresh, to annoy the contestants, or for some other reason, the host will bring new players in after the game has started.
Veterans-Sometiems players from previous seasons join newer seasons midway after originally failing to qualify.