This topic might have a better name.
Ovid (species) to disambiguate from the historical figure/writer Ovid
Talk about it here.
Ovids were beings of pure thought that travelled from world to world in crystal spheres. Four of them became trapped in New York City in the 1890s. They can give form to dreams and nightmares. Harry Houdini helped the First Doctor, Ben Jackson and Polly Wright free the Ovids.
The Fifth Doctor received a message from his eleventh incarnation through an Ovid sphere. (AUDIO: Smoke and Mirrors)