The Meaning of Life, the Universe, and Everything.
I don't believe data packs can add entirely new entities like that - They're more for extending the current functionality - similar to the function of command blocks. You can combine with a resource pack to add in custom textures, sounds, and models, but I think the only way to add a whole new entity is to use a mod.
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Remember those versions that minecraft pranked us with? Specifically:
Those are still downloadable! Watch this video for 2.0:
To download the other ones you need to make a folder in the versions folder for minecraft and put the client and JSON file for the versions in there. They all need to be named the same aside from file extensions. Once you do that, you will be able to choose that version when making a new profile with the minecraft launcher.