Zekrom

Zekrom is a dual-type Dragon/Electric Legendery who acts as a Hero in some media. He also appears as King Zekrom.

Pokemon White
In Pokemon White Zekrom is the Massot and Hero who will be used to fight Reshiram

White Victini and Zekrom
Zekrom appears in the story of the People of the Vale, but does not appear itself until Ash discovers its Dragon Stone inside the Sword of the Vale. It questions Ash about what ideal he is pursuing, and when he answers that he wants to save Victini, Zekrom is satisfied in Ash's ideals and appears before him as an ally. It bursts out of the cave and rescues Juanita's Golurk by stopping Damon's Reshiram's attack.

Pokemon Black Victini and Reshiram
Sometime during the three years he spent traveling the world prior to the events of the film, Damon happened to overhear Zekrom calling to him while he observed the ruined Kingdom of the Vale alongside Mannes. Together, the two of them went down into the caves inside the Sword of the Vale, where they became separated. At the bottom of the cave Damon discovered the Dragon Stone containing Zekrom and reawakened it.

Zekrom under Damon's control With Zekrom's help, Damon traveled to the villages of the People of the Vale and managed to convince them to support him. Its first appearance in the film comes when it helps Damon destroy a giant iceberg that threatens to destroy Glacine's village in a polar region. It later appears in a flashback as Damon tells the story of his travels. When he raises the Sword of the Vale and comes to be opposed by Ash, Cilan, Iris, Carlita and Juanita, Damon summons Zekrom to aid him. It battles Juanita's Golurk, which Zekrom easily defeats. The second time it fights Golurk, it is interrupted by Reshiram, which Ash has discovered and allied with. The two dragons battle after Reshiram drops Ash at the castle, ending in Zekrom's defeat. Zekrom and Reshiram then realize that Damon's actions have unleashed a corrupted Dragon Force that threatens to destroy the world, and both dragons blows away the clouds to reveal this fact to Damon.

Pokémon RéBURST
In here Zekrom is The Main Pokemon and Ryouga's Partner and Burst