well it depends either you or your rival if you chose mudkip its your rival if you chose it, there you go but i'm not sure if any other trainer has it.
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Ruby, Sapphire and Emerald.
Treecko, Tropius, Seedot, Oddish.
If you are talking about the starters..then,U are able to pick from Torchic,Treecko,Mudkip.
In Pokemon Sapphire, there is only one trainer in the game that has a Ludicolo. To find her go to Victory Road and search around everywhere and you will find a pokmeon trainer that has a Nosepass LV 42, Ludicolo LV42 and Medicham LV42.
For Ruby, Mudkip. For Sapphire, Treecko. For emerald I do not know