Full Idea
If the unicorn myth is supposed to be a particular species, with insufficient internal structure to determine it uniquely, then there is no actual or possible species of which we can say that it would have been the species of unicorns.
Gist of Idea
Unicorns are vague, so no actual or possible creature could count as a unicorn
Source
Saul A. Kripke (Naming and Necessity notes and addenda [1972], Add (a))
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.157
A Reaction
Dummett and Rumfitt discuss this proposal elsewhere.