Mew is #151 in the Pokédex and could only be obtained through special events or by using a cheating device like Gameshark, or a game-enhancing device. Mew can also be obtained in the original issues of the Pokémon yellow and blue (US) by “cheat”. If you get a Pokémon early on (before going to Misty’s gym or defeating man to the left of the nugget bridge) that has growl (charmander has it from the start) and an Abra or a Pokémon that can fly but you can’t fly this early in game (you can get an abra in the grass next to the nugget bridge but be careful not to battle the man standing above that grass he’s needed later. After this go to the health center and heal, then go to the top of the nugget bridge and save. Head down to Misty’s gym, go straight and take a right and challenge hit the guy trainer in a pool to the right use growl 6 times on his shelder then defeat him. Go back to the top of the nugget bridge and take a left to engage in battle with that man but press start right when you do and he should have a ❗️ Over his head when you hit teleport (restart if you start the fight) and teleport back to the health center. Then run up the nugget bridge making sure to have a load of poke balls and the last Pokémon you fought was that shelder after using growl on him 6 times, and you should have a random encounter with Mew. Also if you fight a Pokémon that’s low enough in level not to level your Mew up to level 2, it will automatically level up to 100.