They are winged Shy Guys commonly found in large groups divebombing the player. If one is defeated, more will come to replace it. They will only stop if the player's character leaves the room.
Super Mario Bros. 2
They are also in the remakes, Super Mario All-Stars and Super Mario Advance, where they remain the same, except that they got recolored. The green and gray ones got replaced with yellow, and the yellow got replaced with red.