Before the introduction of Galerian Moltres in Gen 8, Yveltel was the only Dark-Type Legendary in the series, making it incredibly useful in certain situations.
There are plenty of Dragon-type Legendary Pokémon these days, but when Reshiram was first introduced in Gen 5, there wasn't nearly as much competition.
Although not quite as popular as its fellow gen 2 mascot, Ho-Oh, Lugia is still an incredibly useful Pokémon to have on one's team; Assuming that the players can find one.
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