Ho-oh and Lugia at Levels 40 and 70 depending on the game, and the Legendary Dogs at Lv 30 and running.
I wouldn't mind seeing Mewtwo around Level 70 back in Cerulean Cave or Celebi around level 10/20 after you get the National Dex(if they have it).
I wouldn't mind seeing Latios and/or Latias(possibly depending on the game) as running pokemon.
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