Where is the TM Iron Tail in Pokemon X and Y?

Iron Tail is not a TM in Pokémon X and Y.

If you want a certain Pokémon to learn the move, it has to be bred from a Pokémon that knows Iron Tail and can get it as an egg-move. Pikachu can't learn it through this method, for example, the only way is by transferring one that knows the move from a Generation V game (Pokémon Black, White, Black 2 and White 2) using the Poké Transporter and Pokémon Bank apps for the Nintendo 3DS.