Full Idea
I think it is not the case that there might have been unicorns. I wouldn't say it is necessary that there are no unicorns, but that we just can't say under what circumstances there would have been unicorns.
Gist of Idea
That there might have been unicorns is false; we don't know the circumstances for unicorns
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 1)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.24
A Reaction
His point seems to be that unicorns are insufficiently individuated by the legends, whereas a typical sample of an actual creature contains everything that will individuate the species.