But this isn't the move transform though, you need to see your target to be able to transform into that target, so I'd hardly call this transform, since Zoroark doesn't see Raikou, Entei or Suicune before it transforms, and the only one it can see before it transforms is Ash.
So this isn't transform at all. Similar Yes. Identical No.
So it's his ability? It's probably a new ability/move that hasn't introduced yet in the anime/games. And you are right, it's not Transform since he changes 3 times in a row without even see the pokemon he changes into.