Like Houndour and Electabuzz.
This is because trainers have used these Pokemon in battle berfore you have obtained the National Dex. If they did not go onto the Pokedex it would kind of go against the point of the game which is.
Gotta catch'em all!
Game Freak and logic don't mix well. But it's the answer you see a lot. Game Freak wanted it like that.
