I was doing the mew teleport glitch ( yeah i know i suck), but i had already fought the boy with the slowpoke so i went and fought the girl next to him, teleported back and went to the bridge, but when the battle came up it was lapras instead, is that weird or does the pokemon change depending on the person you fight?
The Pokemon changes depending on the special of the LAST pokemon you fought. For example: To catch a Ghastly without a Silph scope, use the Mew Glitch, then fight the trainer with the 2 Growlithes on the SS Anne.
You can also catch the trade evolved pokemon with it. Just read the Mew Glitch FAQ for all the information you need for the glitch, including the Specials needed to generate what pokemon.