Fake bitcoins are those assets that use the name bitcoin which is obviously a copy cat of BTC and claiming to be the real one.
Everything that is called anything other than Bitcoin is probably a fake bitcoin.
So Ethereum and XRP are considered fake Bitcoin, then?
No they aren't considered as a fake bitcoin, they have their own unique features and use the foundation of chain network to their assets. They are called alternative cryptocurrency or altcoins so obviously, they aren't considered as fake bitcoins. Innovations are not considered as copy cats or fake things because you made something different than the original doesn't have.