For questions about animals or non-sentient beings that are not found on Earth. These can be anything from dragons to Pegasus to your own made-up creature. They just need to not exist on this Earth.
Mythic creatures are a lot of different things. That's what makes them so interesting. A mythical creature can be anything you want it to be. That's also what makes them difficult. You not only need to know what they are, but you also need to make it clear to your audience what they are. Sometimes you can know what they look like and what they do, but not how they work.
That's where this site comes in. There are a wide variety of questions you can ask about mythic creatures, like, "Are they realistic?", "How big can they grow?", or "How would they affect buildings?" These are just examples of what should be tagged with this tag. This tag is for anything that has to do with a creature not on this Earth.
See also the creature-design tag.