Following the adventures in the Kanto region in the Pokemon tv series, the main characters had some adventures in a place called The Orange Islands. During that period, Brock (one of the three main characters) was replaced by a character named Tracy. When Ash left the Orange Islands, Tracy was replaced by Brock in turn.
Fans had speculated about why this happened for a long time. In an interview, the director and storyboard artists, Masamitsu Hidaka, explained what happened. Hidaka said that the team behind Pokemon realized it was going global, and that people in America would view Brock as a racist stereotype because of his eyes.
To err on the side of caution, they created Tracy, a caucasian, to avoid any controversy. Eventually the writers realized that Brock hadn't offended anyone, so they wrote him back in.