So, I beat Mario on my Wii U VC and I pressed B to start the game over. When I played again no Goombas were there and I was like, "How do I reset the game to normal with the Goombas?" I beat it again and same thing happened. Can someone tell me how to reset it back to normal with Goombas and without the round shelled guys that are not Koopas (don't know their names, lol)? I just got it a day before this post. I really want to speedrun this!!!!

I think you have to delete the game data or save or something. Not the actual game itself.

The other shelled enemies are Buzzy-Beetles

Well, how do I remove the Buzzy Beetles? Better question: can you speedrun with Buzzy Beetles or do you need to have the Goombas? Please answer.

Are you not playing "Second quest"? If you press B for "Select a world" you can be at the Second Quest game mode where every goombas became Buzzy Beetles. This mode is like harder difficulty level and have other different things.

There is a category to speedrun with the buzzy beetles, but I wouldn't mess with it unless you really want to. The timing of the jumps is completely different in some cases because they spawn a little differently. Go back to Goombas imo :)

What category is it? BTW, I found a way to reset to Goombas. You just go to Data Management settings and click on Super Mario Bros. and delete the save data that has your Mii face on it. Not the game, just the save data!! But anyways, what is the category with Buzzy Beetles called?

yeah that's exactly what roopert suggested.

yeah. I guess. So thanks Roopert!!

Probably game automaticly saved your playthrough and think that you already beat the game and make a harder mode without goombas (Second Quest). https://www.speedrun.com/smbce#Second_Quest