Slithy Tove

When the Seventh Doctor criticized Light's Catalogue of Life due to the apparent gaps present in the collection, he listed Slithy Toves as being one of the species Light had seemingly failed to document. (TV: Ghost Light)

Behind the scenes
In reality, Slithy Toves are a fictional species mentioned in the 1871 poem "Jabberwocky" by Lewis Carroll, though their fictional status is never acknowledged within Ghost Light.