Question

How can Zapdos learn defog?

Answer 1

Zapdos can learn the move Defog through DPPt's HM05. If you want to get it to X & Y, you must get a Zapdos from either Gen 3 (and transfer it via Pal Park) or roaming Sinnoh in Platinum and transfer it to HeartGold or SoulSilver, due to not being able to transfer Pokemon knowing HMs. Since Defog doesn't count as an HM in HGSS, you can then transfer Zapdos to Gen5. Then, use PokeBank to transfer it to X & Y.

Source 1 | Source 2

Comments

That's a lot of work :/
I'll pass...

Answer 2

You can only get it by teaching a Zapdos Defog in gen 4 (HM5) and transfer it to gen 5 then use pokebank.

Answer 3

In Gen IV Defog became an HM move. Being that most flying types can learn it Zapdos can to. Also since it was only a HM in that Gen it can be passed onto Gen V then VI. Thus Zapdos having Defog! Most people just hack one into Gen V and trade to Gen VI.

Look here for Zapdos's Gen 4 learn set.