The —, 1963 novel by Iris Murdoch (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
unicorn
(Other definitions for unicorn that I've seen before include "Animal of fable" , "Animal rumoured" , "Imaginary animal" , "Imaginary creature with one horn" , "Imaginary one-horned creature" .)