Question

I haven't seen him use Volt Tackle in Unova.

Answer 1

Because a Pokemon can only know 4 moves at any one time. Pikachu's Volt Tackle has been replaced with Electro Ball.

Though, there are a few inconsistencies in the anime where some Pokemon know more than 4 moves at a time, but it's widely accepted that a Pokemon can only know 4 moves, just like in the games.

Pikachu's moves

^ That's a list of Pikachu's moves, past and present.

Comments

Ash is a competitive idiot....Volt Tackle is much better off than Quick Attack.

It would've been cool to see Pikachu learn Discharge at some point. Shame it didn't happen.

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Yea but.. this is the anime we're talking about, where his Oshawott still knows Tackle.

And besides, Pikachu usually knows 2 Electric type moves and 2 non-Electric ones; otherwise 3 Electric type moves would really limit the anime's graphics when he's fighting against a Ground type Pokemon (case in point: Iris' Excadrill).