Arnold's mother is a minor character from the episode "Hypno's Naptime." She mistook Ash for her son Arnold when the group first arrived in HopHopHop Town, which gives you an idea of how frantic the situation had become.
Arnold had vanished three days before Ash showed up, along with several other local kids. His mother didn't know why—but it turned out a Hypno being used by wealthy locals to treat insomnia had accidentally brainwashed the children with its sleep waves, casting a wider net than intended. The kids got it into their heads that they were Pokémon and spent their time in the park acting like them.
Once Ash and the others figured out what was going on and brought Drowzee to the scene, the psychic Pokémon undid the brainwashing. Arnold and the other kids snapped back to normal and got reunited with their parents—Arnold with his relieved mother.
Content compiled by AnimeList.moe from publicly available sources.
