A ploonet is a planet that originally was a moon. We don’t know of any yet in the universe, but they are likely to exist.
The most likely way a ploonet can form is when planets with moons migrate closer to their star. The Hill sphere is the region around a planet where its gravity dominates other gravitational sources in the system, such as the star. The Hill sphere shrinks as the planet migrates towards its sun. A moon with its orbit near the outer edge of the Hill sphere can then be freed and become a ploonet.
In our system, planets migrated massively early in their history, but it didn’t seem to have resulted in any ploonets forming. When planetary systems form, they likely experience greater orbital instability until orbits settle. Such migrations can bring planets too close together, freeing their moons and allowing them to become ploonets.
Moons can be freed from planets via tidal effects if their planet spins faster than the orbital period of the moon. This is the case with Earth and the Moon, and our natural satellite slowly migrates outwards. It won’t leave us before the aging Sun destroys our system, but elsewhere in the universe, such tidal migration could create ploonets.
However, freeing them from their planets is not the whole story. They can collide with other planets or with their star and be destroyed. To become ploonets, they need to settle into a stable orbit, which isn't guaranteed.