Timothy caught a Dunsparce during a massive swarm of the Land Snake Pokémon and became a pretty smug about it. He organized a race at the local playground, challenging Bucky's Caterpie against his Dunsparce and everyone else's. The match went south fast—Caterpie couldn't keep up, and when Timothy had his Pokémon fly over an obstacle to gain an advantage, Caterpie ate dirt. Bucky bowed out rather than get humiliated further, and a girl called Timothy out for being a jerk about it.
Later, Timothy and the other kids went to catch Team Rocket's "Moltres versus Dunsparce" show. It was terrible—cheap costumes, rough acting, the whole deal. But after the performance, Jessie offered what seemed like an unbeatable deal: trade your Dunsparce for a Moltres. Timothy jumped at it, and the other kids followed without thinking twice. It was obviously a Team Rocket trap. They all got fake Poké Balls and lost their Pokémon.
When Ash and the others found the kids crying, they sorted out the scam. Once Bucky managed to catch his own Dunsparce, everyone—Timothy included—was able to actually play together without the attitude.
Content compiled by AnimeList.moe from publicly available sources.