free to be the greatest
Unfortunately they are hacked, yes.Snip
The games prevent legit mythical Pokémon like Celebi from being wondertraded around.
There are ways of circumventing this, but, yes, it involves manipulating the Pokémon, resulting in hacks.
You can keep them if you want.
Just make sure that you don't trade them in this community here.
But, since you're already here, maybe you can trade for a legit Celebi here in the future?