>>43531610Strictly speaking it's possible, but almost certainly it didn't happen by accident. Someone's either used a seed to determine when and where that would spawn and then hosted it, or just straight up hacked it in, though I've never looked into how that's actually done or whether those Pokémon are provably cheated. I'd probably suggest keeping it for yourself for breeding purposes / as a trophy, rather than trading it before you find out if it's legal or not.
>>43531616For hosting, connect to the internet with Y-Comm, go to a den, and hit Invite Others. People will have 3 minutes to join. When you defeat it, everybody will get a chance to catch the Pokémon, but only the host will keep the 100% catch rate. Other people's raids will appear in the Y-Comm, just select them and join.