Pretty much what it says on a label. I have a monster for my next adventure that has a pretty interesting ring in his treasure. Since it is intelligent, and would be able to use the ring, I don't see why the ring should be lying there in a chest waiting to be looted.
Monster would use the ring, if it only had hands or even fingers. Hold a second, are fingers even necessary in 5e?
For reference, assume it only has grasping / manipulating tentacles like a deepspawn (although it is not one); Said tentacles are very flexible, capable of fine manipulation, and have varying thickness.
Rings have to be worn... just not necessarily on fingers
In the "Beholders: Bad Dreams Come True" section of chapter 1 of Volo's Guide to Monsters (p. 15), it states under the "Treasure" subheading: