Throughout most of the Mario series, the most prominent enemies have been Goombas and Koopa Troopas. Super Mario Bros. 2, however, was originally a non-Mario game, and therefore didn't contain any of the enemies from the first SMB. Instead, the main grunts were Shyguys, little creatures in colored robes and face masks. They were identified in the instruction booklet (the same one that outed Birdo as a transvestite) as members of a club known as the 8-Bits, along with Snifits and presumably Beezos. The only SMB2 enemies to reappear in SMB3 were Bob-ombs, but later games featured other SMB2 villains, and Shyguys showed up again in Yoshi's Island. Even in SMB2, the Shyguys were no strangers to interesting modes of transportation. They drove tiny fire-shooting cars known as Autobombs, and rode on ostriches known as Ostros (misidentified as Birdos in the closing credits). I'm inclined to think these birds were rather perverted, since they'd look up the Princess' dress if she stood on their heads. In Yoshi's Island, Shyguys ride around in submarines and walk on stilts. Super Mario RPG has Shysters, Shyguys from another dimension that bounce on pogo sticks and work for Mack. I understand that there's any area of Paper Mario known as Shy Guy's Toy Box, inhabited by a lot of different sorts of Shyguys, but I haven't made it that far yet.
Shyguys also appear in some of the Mario sports titles. I've never played Mario Tennis, but I understand that a cut scene in the game shows Luigi as totally shocked when he gets a look at what's under a Shyguy's mask. The Shyguy is facing away from the player at this point, of course, so that what's under a Shyguy's mask is still a mystery to the world of Nintendo players.