Mudkip is a small, amphibious, quadrupedal Pokémon. It has a blue body with a light-blue underside. Mudkip has a large head with a blue fin on top and a light-blue tail fin. It has black, beady eyes and orange, star-shaped gills on its cheeks. By using the fin on its head, Mudkip can detect changes in air and water currents as well as bodies of water. Its fin can also point in a certain direction, as seen in the anime. Mudkip is capable of moving or crushing boulders larger than itself. Its large tail fin provides powerful acceleration to propel it through the water.
According to its Pokémon Ranger browser entry, Mudkip is a very docile Pokémon. Mudkip is more commonly found in captivity, though it can be found in swamps and other wetlands. In the wild, it sleeps by burying itself in the soil at the water's edge.
In A Six Pack Attack!, a Mudkip belonging to Professor Birch was featured along with the other starters. They were being admired by Max and May when Team Rocket captured them along with the other Hoenn and Kanto starters. Max found them and told Mudkip and Squirtle to cool the bars with Water Gun, which eventually freed them. It then helped send Team Rocket blasting off.
In Trying to Trounce Torchic, when Ruby was attacked by Sapphire's Torchic, Professor Birch told him to look in his briefcase, to pick up the Poké Ball with a blue Pokémon and to throw it. The Poké Ball released a Mudkip and he nicknamed him Mumu. In the beginning, Ruby was disappointed with him because he wasn't as attractive as his other Pokémon and he couldn't participate in any Pokémon Contest. He tried with different conditions and then decided that Tough Contests would be perfect for him. Mumu eventually evolved into a Marshtomp and then into a Swampert.
"A Mud Fish Pokémon. It has fins on its head and tail. The fin on its head acts like a radar dish to read its surroundings, and its tail fin increases its swimming speed. The gills on its cheeks allow it to move about in water. It's very strong for its size--it can even lift big boulders. It burrows into muddy watersides to sleep. Mudkip evolves into Marshtomp."
This Pokémon was unavailable prior to Generation III.