In D/P Ash had to teach Pikachu how to use Iron Tail. It wasn't a new move, but a move Pikachu showed knowledge of but could never fully utilize the attack. So they practiced through various episodes to master it.
If memory serves me right, it was before the Rustboro Gym in Hoenn that Ash spent time teaching Iron Tail to Pikachu. Just something I had to point out. So yeah, it was in AG series not DP.