Silver was a baby Lugia befriended by Oliver. It appeared in The Mystery is History, A Parent Trapped!, and A Promise is a Promise.
It appears in the anime. The anime doesn't treat legendaries as a "one off", and legendaries hatch as well, but in an unknown way, presumably because they are so rare that their habits are not fully understood. The Lugia Silver presumably hatched as small Pokemon, and will grow to a full size Lugia. It also showed that it had a parent, so it's presumed that Lugias are bred.
There is a hack version of Pokemon Ruby called Pokemon Ruby Destiny that has a baby Lugia which evolves with friendship, but baby Lugia does not appear in any official Pokemon games.