From the Super Mario Wiki
A Roto-Disc in rotation.
Roto-Discs, formatted as Rotodiscs in the manual of the NES version, are Firebar-like entities that first appeared in Super Mario Bros. 3. As their Japanese name states, they are meant to represent cookies. In this game, one or two Roto-Discs are found orbiting gray or red orbs, on which it is possible for Mario or Luigi to stand. Roto-Discs are nearly invulnerable, they can only be destroyed if Mario or Luigi transform into a statue by using a Tanooki Suit and land on top of one. If there are two discs orbiting one orb, the player will only need to destroy one and the other will disappear.
Aside from Super Mario Bros. 3 and its subsequent remakes, the only other appearance of Roto-Discs is in the Mini-Mario course in Bowser's Castle in Mario & Luigi: Superstar Saga. As a throwback to the previous games, they are obstacles that can kill Mario in one hit, sending him back to the beginning of the area.
Super Mario Land also contains enemies that orbit a similar-locking block and share the same attack pattern; their name isn't given, but it is assumed they are Roto-Discs.
Super Mario Land 2: 6 Golden Coins contains an enemy known as a Guruguri in Mario Zone, which behaves in a very similar fashion to a Roto-Disc.
Names in other languages