This bad guy seems to show up just about everywhere. Vault over it, or go ahead and stomp it flat!
The Goomba is the most common enemy in the Mario games, and is encountered right from the start.
If Mario approaches it, he will automatically vault over, but will cause Mario to lose a life if you hit one from below. Pressing jump when vaulting over a Goomba will cause Mario to do a special jump, and will kill the Goomba.