I dont' know how you received an egg through wonder trade since you haven't ever been able to send eggs through wonder trade (And I'm speaking since Oct 22, 2013 when someone was trading directly with me through trade and of the pokemon they were showing me, one was an egg, so I tried to wonder trade an egg as soon as I could and EVERY time I try the game ALWAYS says "You can't offer an egg for trade." both through wonder trade and through the GTS, so I think receiving an egg through wonder trade is as likely as the people who say they received a Mega Stone through wonder trade.
I've traded many eggs before. eggs are tradeable in person to person trade, of course not on wonder trade or gts. but normal eggs have absolutely no relation to a bad egg other than the similar appearance. Anyways, you get Bad Eggs on wondertrade as an extra hidden surprise in your PC boxes, you of course get the pokemon it showed you receive in the trade as well.
here's something a bit more informative about bad/mystery eggs in gen VI:
"Mystery Egg" appears to be (the product of) an error handler for invalid Pokemon data, in much the same way that
Generation I's "MISSINGNO." name was (the product of) an error handler for Pokemon species that had zero-length names. The "Mystery Egg" error handler exists in order to handle invalid Pokemon data -- to supply various parts of the game engine with valid data, so that things don't break.
Accordingly, it seems reasonable to assume that Mystery Eggs can never hatch, or that they will simply hatch into other Mystery Eggs (akin to
Species 253 in
Generation II). If they are intended as error handlers and safe wrappers for invalid data, then why would they hatch into anything else?
Mystery Eggs appear to have been acquired from the "
Mystery Zone" on "0/0/2000". This is likely because the relevant data is either null (zero), or is being treated as null by the "Mystery Egg" event handler. "Mystery Zone" is yet another error handler: a placeholder name given to maps and locations that don't have their own names. You could therefore read the Egg's origin as: "obtained from nowhere on never."
So if Mystery Eggs are simply safe wrappers for invalid data, then why would they multiply? Well, on a stable and undamaged copy of the game, they wouldn't. The most likely explanation is that some individuals have damaged copies of
Pokemon X and
Y that are creating invalid Pokemon in the PC, likely by mistaking empty PC slots for filled ones. The parts of the game that are still functioning properly catch the resulting invalid Pokemon and handle them as Mystery Eggs.