Alan Rhoda argues as follows:
'In one sense of the word, to say something "exists" is to say that it is actual or real. But that can't be the sense implied when we say that something "is" possible but non-actual or that something "is" impossible because both of those categories exclude actuality. So we have to recognize at least one additional sense of "exists" besides "is actual"'.
Do we have to? I'm sceptical. Here's Alan's argument in a nutshell.
(A) Things such as unicorns are possible, though there aren't such things
(B) There are some things which are possible, though there aren't such things
I suppose we have to admit (A) for the same reason we have to admit 'Some of Jane Austen's characters are working class'. But (B) seems to imply
(C) There are such things, such that there are no such things.
I'm not sure I want to admit that. Certainly Meinong said 'Those who like paradoxical modes of expression could very well say "There are objects of which it is true to say there are no such objects'. But then he would, wouldn't he? And it is paradoxical.