It sleeps in a deep-sea trench. If it flaps its wings, it is said to cause a 40-day storm. (Diamond)
Well, considering Lugia's primarily way of getting around is "flying" - and the way it flies is by flapping its wings - Lugia's causing years worth of storms pretty much every time it goes anywhere. The Pokemon world should be covered in an unceasing meteorological nightmare, all because one stupid legendary Pokemon can't just sit still for its entire life.
As it moves, it loses bits of its body from which new Grimer emerge. This worsens the stench around it. (Gold)
Grimers are infinitely self-replicating - which would be pretty nightmarishly-powerful on its own (additionally Muk, Grimer's evolved form, vomits up Grimers) - but, for some reason, this also makes it smell worse and worse. Eventually, it should have untold numbers of asexually-produced children (who, in turn, are leaving behind their own untold number of asexually-produced children) AND an infinitely horrible stench around it.
It is described in mythology as the Pokémon that shaped the universe with its 1,000 arms. (Diamond)
Hey, this Pokemon here? Of the same genetic family as a psychic mime and a sentient bag of garbage? It's basically (literally) GOD. Also, it somehow has 1,000 arms, none of which can be seen. Maybe it got rid of them because it's GOD and doesn't need 1,000 arms because it apparently can get along just fine shaping the entire universe with a couple legs.