There are 2 Pokemon in the game that you can steal.
1. The first one is given to you by a guy in the Goldenrod Gatehouse. The pokemon is Spearow and it will have mail. Take this mail and put it in your PC. Then, get a weak pokemon (i.e. Magikcarp) and give him the mail. Then, go to a sleepy guy. He will say "What? You have mail for me?" then, select the pokemon with the mail and the guy will take it. You loose that pokemon, but the Spearow is yours for the taking.
2. The second one (Shukel) you get from a scardy-cat in a city by the water. Take the Pokemon, but don't return it.

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