There is only ONE way to get a lucky legendary pokemon. If you and a trade partner get a guaranteed lucky trade, it's possible to trade legendary pokemon and have them be lucky. I know, I've done it multiple times already. I have 6 legendary luckys

I had trade & doesnt produce lucky.

my lucky poke only 8 (delete my lucky poke to 8, before have 12)

& I trade electabus (30/7/2016) with dialga

nor can you get a darkrai for a lucky trade which is completely mind boggling

Meltan are Mythical but you can still trade them so you can certainly get a lucky Meltan. I have both Lucky Meltan and Lucky Melmetal.

The other Mythical Pokémon are not tradeable, so as of right now (23/4-19), you can't get a lucky Mew, Celebi or Deoxys.

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