I. EXISTENCE
2. BEING, IN THE CONCRETE
Unsubstantiality.
[Nouns] unsubstantiality, insubstantiality; nothingness, nihility; no degree, no part, no quantity, no thing.
nothing, naught, nil, nullity, zero, cipher, no one, nobody; never a one, ne’er a one; no such thing, none in the world; nothing whatever, nothing at all, nothing on earth; not a particle (smallness) [more]; all talk, moonshine, stuff and nonsense; matter of no importance, matter of no consequence
thing of naught, man of straw, John Doe and Richard Roe; nominis umbra, nonentity; flash in the pan, vox et praeterea nihil.
shadow; phantom, (fallacy of vision) [more]; dream (imagination) [more]; ignis fatuus (luminary) [more]; ” such stuff as dreams are made of ” [Tempest]; air, thin air, vapor; bubble [more]; ” baseless fabric of a vision ” [Tempest]; mockery.
hollowness, blank; void (absence) [more].
inanity, fool’s paradise.
[Verbs] vanish, evaporate, fade, dissolve, melt away; disappear [more].
[Adjectives] unsubstantial; baseless, groundless; ungrounded; without foundation, having no foundation.
visionary (imaginary) [more]; immaterial. [more]; spectral [more]; dreamy; shadowy; ethereal, airy; cloud built, cloud formed; gossamery, illusory, insubstantial, unreal.
vacant, vacuous; empty. [more]; eviscerated; blank, hollow; nominal; null; inane.
[Phrases] there’s nothing in it; “an ocean of dreams without a sound” [Shelley].
Time's Flow Stemmed
Wild reading . . .
Time's Flow Stemmed 
Years ago I read an interesting book, The Book of Nothing by John D. Barrow, an account of how the zero came into being in ancient civilisations. In it he discusses the work of Otto von Guericke, a German scientist who conducted experiments in 1654 relating to the vacuum:
“For Von Guericke everything that existed could be put into one of two classes: it was either a ‘created something’ or an ‘uncreated something’. There could be no third way: no class that we can call ‘nothing’. Since ‘nothing’ is the affirmation of something and the opposite of something else, it must be a something. Thus it falls into the category of either the ‘created somethings’ or the ‘uncreated somethings’; or maybe, he feels, ‘nothing’ has a call on belonging to both categories. Thus an imaginary animal like a unicorn is nothing in the sense of being non-existent; that is, it is not a thing. But because it exists as a mental conception it is not absolutely nothing. It has the same type of existence as a human thought. Thus it qualifies as a created something.”
Weird, but kind of fascinating, don’t you think?
Curiously enough I read that same fascinating book, years ago. Part of my literary history includes a Robert Heinlein phase (slightly embarrassed about that); he plays with this idea in one or two of the books, which deal with the status of fictional characters that occupy the same space as your unicorn.