According to the Tenth Doctor, there were representations of a horned Beast in the myths and legends of a million planets across the universe. (TV: The Satan Pit)
The Shadow Architect believed that the Time Lords belonged in the "myths and whispers" of the higher species. (TV: The Stolen Earth)
The Eleventh Doctor believed the Shakri to be a myth to keep young Gallifreyans in their place. (TV: The Power of Three)