Satchel was a trainer that went to participate in the Johto League. The heroes met him and Ash had a battle with his Magby, using his Cyndaquil. However, Magby overpowered Cyndaquil and defeated it. Later, Team Rocket stole the Pokémon, including Satchel's Magby. However, Ash and Larvitar freed it and, during the battle with Team Rocket, Magby evolved into Magmar, making Satchel pleased.
As a Magby it was seen battling with Ash's Cyndaquil and it eventually defeated it. During the same episode Team Rocket captured it and other Pokémon. This is when it evolved into Magmar.