What are some playable prototypes that were completely different than the final game?

Game development can be an interesting thing. 3D Realms' Prey looked completely different in 1995 compared to the final release, same with Duke Nukem Forever, while others like The Last Guardian never shifted focus too much, but there are still somethings that shifted around as development went on. I mean, it skipped a whole console generation.

But what I want to talk about to today are the prototypes of that that somehow made its way into players' hands. Be it official demos, or leaks on the internet, which prototypes of any released game, show a different take on what could have been in the final product if it came out earlier?

Prototypes of unreleased games are welcome too, but I don't know how to squeeze them into the question.

Ubisoft games

Team Fortress 2, DOOM 4. Both games had massive changes, but neither early prototype was playable by the public. Halo is another one.

Halo, I believe it was originally going to be a RTS game.

Also a lot of Resident Evil games. RE2 was scrapped after about being 75% done and completely remade, same with RE4. And Devil May Cry started out as a RE game but it was to action oriented so they made it into DMC.