Brock's Mudkip lived in a preserve run by Old Man Swamp. Brock first met the Mudkip after they both helped rescue a baby Mudkip on the preserve. Later another baby Mudkip fell off the cliff and Brock's Mudkip managed to rescue it but was caught on a rock. It's trust in Brock increased after Brock rescued it from the waterfall and also when they worked together to fed off Team Rocket.
After rescuing the baby Mudkips from Team Rocket, Old Man Swamp noticed how well the older Mudkip got along with Brock and he suggested that it should journey with Brock.