Mega evolutions only require a basic pokemon

Octowen

https://community.pokemon.com/en-us/discussion/20218/mega-evolutions-only-require-a-basic-pokemon

Early iterations of the mega evolution cards did not require a fully evolved Pokémon in play to mega evolve. They evolved from the EX form of the same Pokémon, e.g. Lucario EX > M Lucario EX.

(This process would end you turn if you did not have a Spirit Link attached to the EX.)

Hades0918

https://community.pokemon.com/en-us/discussion/comment/98816#Comment_98816

Right

And EXes were always basics

But you couldn't just evolve them

There was a proper system