UPDATED
Several Pokémon are often disputed as to whether or not they are truly Legendary or are often mistaken as true Legendaries by fans.
*scroll to end for note
Rotom - Rotom is often thought of as Legendary because there is only one in a given game and it plays the Legendary Pokémon music once battled. It is also labelled as a Legendary in the DP Pokédex Book, although the same book bears several other errors (numbering in the double digits), both major and minor.
Arcanine - Arcanine is commonly misconstrued as a Legendary Pokémon, mainly because of an episode (named "Pokémon Emergency!", and known to be the second episode of the show) which showed Arcanine as one of the four Legendary Pokémon on the tablet at the Viridian City Pokémon Center, and because its species description since Generation I has been listed as the "Legendary Pokémon". Despite this, it is not a true Legendary.
Unown - Unown is believed by many to be a Legendary Pokémon because it is shown in Molly Hale's book about Legendary Pokémon in Spell of the Unown, its power in large number appears to match that of Legendary Pokémon and within the canon of the movie is explicitly referred to as a Legendary Pokémon.
Riolu & Lucario - Lucario is thought to be a Legendary Pokémon due to the fact that it starred in a Pokémon movie, which is usually reserved solely for Legendary Pokémon. Another reason is due to the fact that at the time of its release the only way to get a Lucario was by evolving a Riolu which was only obtainable through an in-game event, and thus was only obtainable one time in the game.
Zorua & Zoroark - Zoroark was thought to be a Legendary Pokémon because, like Lucario, it was the main focus of a movie, a feat normally given only to Legendary Pokémon. Like many other Legendary Pokémon it was only available as an in-game event in Black & White and Black 2 & White 2 with only one being able to found in each game. In Black & White, a trainer needed an event shiny Raikou, Entei or Suicune in order to activate the in-game event and in Black 2 & White 2 it is only available by evolving N's Zorua. This might also be because Zoroark can be freed from a Master Ball in Super Smash Bros. 3DS, Wii U, and Ultimate, something usually only given to Legendaries or Mythicals.
Volcarona - Many people speculated on whether or not Volcarona was a Legendary Pokémon or not because of the fact that it can only be found one time in a game just like Legendary Pokémon and is found through an in-game event. Volcarona is found at the bottom of Relic Castle and is only accessable after the player has defeated Team Plasma in Black & White.
Delibird - The DP Pokédex Book incorrectly states that Delibird is a Legendary Pokémon. This confused many fans and was either believed to be true or was put off as an error, Similar to Spiritomb, Rotom and Spinda, this is just an error out of many in the DP Pokédex Book.
Spinda - The DP Pokédex Book incorrectly states that Spinda is a Legendary Pokémon. This confused many fans and was either believed to be true or was put off as an error, Similar to Spiritomb, Rotom and Delibird, this is just an error out of many in the DP Pokédex Book.
Spiritomb - The DP Pokédex Book incorrectly states that Spiritomb is a Legendary Pokémon. This confused many fans and was either believed to be true or was put off as an error, Similar to Rotom, Spinda and Delibird, this is just an error out of many in the DP Pokédex Book.
Absol - It is said to foreshadow bad luck when it appears and is very mysterious. It also doesn’t evolve and bears a slight resemblance to the legendary beast trio.
*but let us not forget the movies where a clear nod towards multiple legendary pokemon exsisting for example the famous rayqayza from the deoxys and rayqayza movie (i dont remember the movie) and the literal shiny mega rayqayza in the gen 6 movie (i also dont know its name as i never seen it) as well as lugia being a mother 2 the baby lugia which we still question where it came from sorry to say it folks but this theory of mistaken legendarys is quite flawed aside from the mons who are clearly not legendary pokemon because just cuz we havent seen multiple versions of the same ledgendary pokemon in the games or the anime by logic it doesnt mean they dont exsist i recemend looking up said movies i was talking bout if ya want proof i mean how else would lugia have a baby to take care of and who the father lol just saying -jdarkheart ps this just needed to be said you can be mad if ya want
