How do Medallions work?
The short answer is “Magic” and don’t think about it too hard. Medallions cast a complex illusionary spell on the wearer to make them look like a human. There are different medallions for each species of mythical creature, and each can only cast one spell at a time (so no sharing medallions). They don’t work on humans. I think that might answer all the questions I was asked about Medallions, hah.