I once saw a video on YouTube where someone had a Lugia that knew the move Psycho Boost. I was very confused when I saw that, because (as far as I know) Psycho Boost is the signature move of Deoxys. Was that Lugia hacked or is there a way to get a Psycho Boost Lugia in earlier Gens? Maybe it could be an event move as well.
As you can see, Lugia doesn't learn Psycho Boost in any way of level up, TM, HM or Tutor move (and Egg move of course but Lugia can't breed lol):
And as shown here, the only Pokémon that is able of learning Psycho Boost is Deoxys:
My guess is that there was an event for this move on Lugia. Does anybody now?