Stephanie's father brought his daughter to the local Pokemon Center where Nurse Joy had 3 starter Pokemon ready for his daughter to choose.
Combusken evolved and was not available as a starter Pokemon for his daughter. When it finally came time for his daughter to choose a starter Pokemon she chose Mudkip.
Treecko and Stephanie's Father liked each other from the time they met and he asked Nurse Joy if he could keep Treecko.