This may be a gen 2 pokemon but it's not a Johto pokemon as its not native to Johto, it is native to Kanto and Hoenn however, it was only recently discovered during gen 2 but could only be found in Kanto and it is found quite commonly in Hoenn as your rival Brendan or May uses one if they didn't pick the fire starter and Flannery uses two Slugmas in her gym battle.

Also for example, this would mean Pidgeotto is a Johto and Kanto pokemon and Noctowl is a Sinnoh and Johto pokemon.