If you are attuning to a magical ring the DMG magic item categories states:
Magic rings offer an amazing array of powers to those lucky enough to find them. Unless a ring’s description says otherwise, a ring must be worn on a finger, or a similar digit, for the ring’s magic to function.
However, the DMG section Wearing and Wielding Items states
Using a magic item’s properties might mean wearing or wielding it. A magic item meant to be worn must be donned in the intended fashion: boots go on the feet, gloves on the hands, hats, and helmets on the head, and rings on the finger. Magic armor must be donned, a shield strapped to the arm, a cloak fastened about the shoulders. A weapon must be held in hand.
This appears to contradict the previous ruling, so what would qualify as a "finger or a similar digit"?
Further on, it states
In most cases, a magic item that’s meant to be worn can fit a creature regardless of size or build. Many magic garments are made to be easily adjustable, or they magically adjust themselves to the wearer.
That leads to the question, could a toe ring work? We then move toward gifted male* adventurers, pierced horn / tusk / frill / nose / ear, etc
*male is assuming anyone with traditionally male genitalia