From Bulbapedia, the community-driven Pokémon encyclopedia.
A version mascot is a Pokémon that appears on the boxart—and often label—of one of the Pokémon games in the main series.
Early on, the first two version mascots in a generation shared more of a relation with each other than with the third; however, in the Hoenn and Sinnoh trilogies, all three had a significant relationship between them.
Ho-Oh and Lugia
| Version mascots of Gold and Silver
Pokémon Battle Revolution
Pokémon Ranger: Shadows of Almia
Images of the various version mascots and their boxart can be found on the Bulbagarden Archives.