Stephanie's Mudkip was the Pokémon she chose as her starter Pokémon. Stephanie's father brought her to the local Pokémon Center where Nurse Joy had 3 starter Pokémon ready for her to choose from.
The starter Torchic that Nurse Joy had ready for Stephanie to pick from escaped from Max and Ash and evolved into a Combusken. With the evolved Torchic not available as a starter Pokemon for her to choose from, she chose Mudkip without hesitation anyhow.