Old Man Swamp is a man who runs a shelter for Mudkip and resembles the Pokémon he takes care of. He receives Mudkip Eggs from the Pokémon League and raises them to be starter Pokémon for beginning Trainers of the Hoennregion. When he saw that Ash and company trespassed in his secret shelter grounds, he thought that they were trying to steal the Mudkip. Soon everything was straightened out when Ash explained how Brock saved a baby Mudkip. But Brock explained to him later that another, older, Mudkip helped rescue the baby Mudkip.
Old Man Swamp has a tendency of giving out too much information. One instance was him telling Team Rocket not to break the dam because it would flood the area and the Mudkip would have nowhere to live.
When Old Man Swamp saw how Brock and Mudkip worked together to save the baby Mudkip and stop Team Rocket, he eventually asked him to take Mudkip with him as it was getting too old to give to a beginning Trainer. Both Brock and Mudkip happily agreed.
Egg → Mudkip (multiple)
Old Man Swamp receives many Mudkip Eggs from the Pokémon League and raises them to be starter Pokémon for beginning Trainers of the Hoennregion. One day a baby Mudkip falls into the water and is washed away, Brock used his Lotad to save it but then needs help and a bigger Mudkip helps them back to shore. Later Ash and his friends witness the birth of a newly hatched Mudkip. Team Rocket suddenly appear with their giant robot and have destroyed the dam, letting the water flood the river. Brock sends out his Forretress to keep the water at bay while he takes the Mudkip to higher ground. Team Rocket then come with their robot and scoop up all the Mudkip in a big bag, but are defeated by Lotad and Mudkip.
This Mudkip did not leave the pond as it grew older, and was already mature and acted as a guardian for the younger, smaller Mudkip. It joined Brock and left the pond after he helped it rescue itself and a younger Mudkip from a Team Rocket attack.