BUR #4162 has been approved by @evazion.
mass update beta_pokemon -> prototype
Admittedly I created beta_pokemon tag at some point, but that's really just pokemon prototype.
Or pokemon_(creature) prototype if we don't want humans.
Also the word "beta" is misleading because these Pokémon don't come from any official beta version, they are just prototype (that is, unused) designs.
The Cutting Room Floor (TCRF) wiki also seems to use the word "prototype" rather than "beta". I've been using that website when I was organizing / editing tags and wikis for these unused Pokémon designs.
For example:
https://tcrf.net/Proto:Pok%C3%A9mon_Gold_and_Silver/Spaceworld_1997_Demo
Previously discussed in topic #16725.
Updated by Danielx21