Celebi

While trying to escape from a Pokémon Hunter, Celebi stumbled across Sam, who was out looking for Pokémon. Just as the hunter was about to capture the Pokémon, it teleported with Oak and ended up in the time of Ash and his friends.

However, there was still danger in the present, this time from a Team Rocket member known as the Iron-Masked Marauder. Using a Dark Ball, the Iron-Masked Marauder captured Celebi to use for his own evil plans. With the help of Ash and the others, Celebi was set free from the Iron-Masked Marauder. However, it looked very grim for Celebi, who wasn't able to heal itself in the lake where it had healed before.

Fortunately, many other Celebi from the past and future came to heal their friend. Once Celebi was healed, it was able to go back to the past and take Sammy with it.

Celebi also made an appearance in Pokémon Chronicles. In Celebi and Joy, Celebi took Ritchie back in time to help stop the destruction of a Pokémon Center.