When you get all 16 gym badges, go to Saffron City and enter the main building (the biggest one in the middle). There you will see Steven. Make sure you have an empty space in your party and then talk to him and select "blue stone", and VIOLA! you have a mudkip.
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you have to get mudkip
you have to get it as a starter to get it
You can find wild Mudkip in Union Cave. They're just really, REALLY, REALLY rare though.
mudkip
In the new Pokemon HeartGold game, you can catch new Pokemon such as Beldum, Latios/Latias and you can choose between Treeko, Mudkip, or Torchic. (Can all be obtained through Pokemon League Champion Steven from the Hoenn Region.)