Does a Mythical Pokémon count as a legendary?
In short, Mythical Pokémon are a subset of Legendary Pokémon. A Legendary Pokémon is characterized by its rarity, its inability to spawn offspring of its own evolutionary chain, and its presence in the lore.
Why isn’t lucario a Legendary Pokémon?
Lucario and Zoroark are mistaken as pseudo-Legendaries because of the way in which they are to be obtained. Lucario can only be obtained in Diamond and Pearl on Iron Island when Riley gives the player a Riolu Egg. Zoroark is, as of yet, the only non-Mythical Pokémon that can only be caught via an event.