Limited prints of cards that have themed arts, that eventully get in printed in sets with MTG themes later.
So you might have a Secret Lair with the theme being Godzilla as a legendary creature, you can buy online for a premium or you can wait until they print the exact same card in a set as Big Dino #116 with the same text except name and without the Godzilla art.
Secret lair stuff?
Regular releases. If you hop on Arena right now everyone is running Avatar the Last Airbender cards because they're standard legal.
Limited prints of cards that have themed arts, that eventully get in printed in sets with MTG themes later.
So you might have a Secret Lair with the theme being Godzilla as a legendary creature, you can buy online for a premium or you can wait until they print the exact same card in a set as Big Dino #116 with the same text except name and without the Godzilla art.