In Pokémon White, if the player doesn't capture Zekrom at N's Castle, due to having a full party and full PC Box,
Zekrom can be encountered at the highest level of the Dragonspiral Tower after becoming Champion.
In Pokémon Black 2, after the player defeats N in the ruins of N's Castle, the player will be given a Dark Stone.
When the Dark Stone is brought to the peak of the Dragonspiral Tower, Zekrom will awaken, and can be challenged to a battle.
Zekrom cannot be encountered shiny in Generation 5.