If it's Gen VI, you can soft reset. All Gen VI caught lengendaries will have 3 perfect IVs, so if you have the patience & time, you can soft reset until you get one with 4 or 5 perfect IVs.
Before Gen VI, they were definitely Pokegenned. Not saying they aren't in Gen VI, just saying that it's actually a possibility to obtain a 4 or 5 IV legendary legitimately.