Its #722 and can be created via Ram hacking or from pokemon data becoming corrupt. The game makes one as a defence much like a quarantine.
You cant just trade them but you can trade a pokemon that then becomes corrupt (and turns into one) but that's not the trades fault or person trading its an issue with the pokemon being stored on your side.
When a pokemon is sent via p2p, wondertrade or GTS the checksum is checked and if its incorrect data it will fail (if this was not the case we would have a working system inject pokemon right now).
If you trade a Mew for a Ditto it has to come out as a Mew or a Ditto it can't turn by magic into a bad egg as the trade will check to see if data is all good (like if you download say a windows ISO and check is hash is the same to make sure not corrupt)
If it was possible to ignore the checksum or fake it we would be injecting fake pokemon right now along with cloning. They are a result of corrupt data or from you doing a ram hack there should be no other possible way to get one.
If you do see some of these showing up i would send your game back to the store you got it from as its got issues (could be the 3DS its self but likely the game at fault) if the game is digital change SD card as thats most likely the cause.
