Rumor has it that the brown, mushroom-shaped, henchmen were supposed to be the Koopa Troopas in the beginning of ‘Mario Bros. NES’ (a.k.a. the popular turtle henchmen) but play testers had stated that the Koopa Troopa seemed too tricky of an enemy to square off with straight away. Thus, developers decided to create an enemy that could be defeated with a single stomp and so the Goomba was their solution.
And yes, you read that correctly, people thought the Koopa Troopas were too difficult an enemy to begin with. Insanity.