Hajime

Arnold is a character of the day who appeared in Hypno's Naptime. He is a doppelgänger of Ash, with the only differences being his clothing and hair color.

He was one of the children who became hypnotized by Hypno's sleep waves, resulting in him becoming lost at the park. Luckily, Drowzee's dream waves were able to wake him and the other children up. At the end of the episode, he was reunited with his mother.

(Source: Bulbapedia)