Mythical creatures like unicorns, pegasus, dragons and so many others are a great source of detail for your world.
This is not a prompt for you to write a new species but to write the Myth or legend surrounding that creature.
Even if your world is filled with Dragons, the chances are that most people wouldn't have seen them. Most people do not survive the encounter.
What are the stories told about dragons in your worlds, the rumours spread.