Arnie's Brother is from Hamtaro: Ham Ham Heartbreak. He lost his toilet training seat, and his big brother Arnie was searching for it. Arnie's brother looks a bit like Oxnard from behind.
Later on in the game, as you walk into a cave, he switches places with Bijou. When you try to interact with him, he starts to cry. You first need to calm him down before you follow him through the cave to take him back to Arnie's house.