Tumblesets (Clowny in the Japanese version) are obstacles/enemies in scene 4-1.
Tumblesets roll on yellow-green wheel. They wear red overalls with red and white striped arms and legs and have a small blue hat. Their bodies are attached to the wheel part by these blue cubed objects to keep them still.
The Tumblesets don't intentionally try to hurt the player but when you're trying to set up the beginning of the stage, they can roll into you and hurt you. Just try to dodge them. Their real purpose is to help the player on top of them.