okay recently thres been some contreversy over wether or not the pokemon lucario was a legendary. well i am now going to answer that question for you with the best avalable evidence . if nintendo comes out and contradicts me fell fre to ignore me.

i have to say its not a legendarythere are a few spesific requirements to be a legend
1 it has to be hard to obtain in game meaning it is either a roaming pokemon, at the end of the dungeon, or in an area only acesable through nintindo event.
2 it has to be one of a kind. meaning ou wont ever seeit being used by an npc trainer and lucario is used in veil stone gym and by that dude on iron island who gives you the riolu egg.
3 it has to have in game mythology like the legendary birds dogs golems and elves did and mew mewtwo lugia ho-oh celebi kyogre groudon rayquaza dialga palkia garntina heatran cresalia darkrai shaymin & arcerus.
4 legends dont evolve

see lucario is hard to obtain but he isnt unique has no in game mythology and it evolves from riolu. while lucario is incredibly rare ad did devlopi its own legend in the town in the movie but thats just because lucario was the pokemon of their savior. if alakazam or gardeivor or gallade was aaron's pokemon then one of them would be the towns legend. so lucario is not a legend

likewise deoxys is not a legend because it has no in game mythology. it is simply n insanely rare and owerful pokemon.

manaphy and phione meet legendary requirement. they are incredibly dificult to obtain. they have the sea prince mythology. they are unique to you in game and while manaphy does breed which makes it a uniqe legendary. neither form evolves.

that should clear up any questions bout egendary pokemon legendary pokemon
these requirement dont come from nintendo i just found 4 guidelines by comparing what made each games legendarys legendary.