Lucario is not obtainable in any generation before Sinnoh.
I believe that Lucario is shown because he made an appearance in Pokémon: Lucario and the Mystery of Mew which debuted on
July 17, 2005(japan) September 19, 2006(USA).
The movie happend during the season of Pokemon Advance (Nov 21, 2002 to Sep 14, 2006)
Hoped I helped!