Zekrom is a wild dragon/electric-type Pokémon who appeared a couple times in the anime.
Zekrom first appeared in the first episode of the Best Wishes series, In the Shadow of Zekrom!, when Ash and his Pikachu first arrived in the Unova region. Team Rocket attempted to steal Pikachu from Ash until Zekrom sends its bolt of electricity on them. During which, it zapped Pikachu and literally made it unable to use its electric attacks. It appeared again in Enter Iris and Axew!, when it helped Pikachu regain its attacks. Soon after that, it was not seen again but it was mentioned a couple times as the series progresses.