The Creeper's Code is a minecraft datapack that intends to add features that mesh seamlessly with the existing 'vanilla' minecraft content.
Follow the download steps found at https://thecreeperscode.com/download/.
This above page also lists valid versions and alternatives.
This data pack currently adds 20 mobs to the game. In the final release there will be 3 new bosses, 6 new structures, a plenty of new items and a total of 60 mobs. You can find out more by visiting our official wiki or watching this video:
This data pack has not been updated to 1.20 yet, so...