Goomba

“The Goomba was the last enemy created during the development process of Super Mario Bros. despite being the first encountered. During development, the only basic enemy was the Koopa Troopa.

However, play testers had stated that the Koopa Troopa was too tricky of an enemy to have as the first one. Using this, the developers decided to create an enemy that could be defeated with a single stomp. 

The Goomba itself was created with the appearance of a shiitake mushroom. Since the enemy was created toward the end of the development cycle, though, hardly any bytes remained on the cartridge. Therefore, to give the enemy moving animations, the developers reused and flipped the image, making it appear to be walking“

Source: Super Mario Wiki