I've been playing with and/or against Darmanitan in Pokémon White 2 for a while, and I've never seen Darmanitan change into Zen Mode. Was it introduced in Generation VI?
Darmanitan's zen mode was introduced in gen 5 as a hidden ability. You can catch it after you complete the main story by using rage candy bar on statues.
Darmanitan has zen mode in gen 5