This is a custom sprite version of the Boo Ring. It is more efficient than the vanilla version and quite customizible. You can set number of Boos, space between each ghost, and rotation direction by just setting the extra property bytes and extra bits. See readme for more info.
Note for advanced hackers:
This sprite uses the WRAM write registers (0x2180-0x2183) for OAM transfer. If you use these registers during an interrupt, there might be a crash. If you don't know what this means you probably don't have to worry about it.
@GHB: Wouldn't be doable if you made a boo ring that's made from normal sprites; there are clearly more than 12 boos on those screenshots. The sprite runs a custom hitbox routine which seems to take the position of each boo and store their information on stack to work with that.