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English Answer:
Riolu is not considered a legendary Pokémon. Legendary Pokémon are typically characterized by their rarity in the game, their unique stories, and often their immense power. While Riolu is a rare Pokémon that evolves into the powerful and highly regarded Lucario, it does not fall into the category of legendary Pokémon. Lucario, however, is often mistaken as a legendary Pokémon due to its popularity and the fact that it was one of the mascots for the fourth generation of Pokémon games, but it is not classified as one either. The distinction between a legendary and a regular Pokémon is that legendary Pokémon are usually involved in the game's lore as powerful or mythical creatures, often with a single entity existing in the game world, whereas Riolu and Lucario do not fit this description.

中文回答：
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Lucario = Legendary, Even though Maylene of the Veilstone Gym had a lucario, They created a whole pokemon Movie (Pokemon Lucario and The Mystery Of Mew) about him. My opinion, he is legendary. Riolu = Not A Legendary, In pokemon D/P/P, if you bread a lucario, the egg turned out Riolu.Apr 29, 2009read more >>