Level 70 Fug set an impossibly high standard that people think every legendary is up to speed with when it's not the case, although it still usually is a very strong catc that's not underleveled. Not that rare of an occurence in the franchise, actually, although you really gotta search for them.
Best example is probably the Level 50 Magikarp in HGSS. If you have a Pokémon at Level 25, you can use a Repel on the pond before the Lake of Rage to catch a Level 50 Magikarp. Give it a Rare Candy, and you got a free Level 51 Gyarados.