Goombas aren't just iconic staples of Nintendo's Mario universe, they're the first enemy players came up against in Super Mario Bros. That wasn't always the case, though. According to the first-ever Know Your Enemy video from GameXplain, goombas weren't originally planned for the game at all.
The famous mushroom-heads were conceived of as a teaching tool so that players could organically discover that the key to defeating enemies was jumping on their heads without having to deal with the "tricky" koopas. There's plenty of other interesting details about the history of goombas in this video, so if you ever want to study up for a game of video game Trivial Pursuit, here is a good place to start.
The more you know!