For the record, I've always hated the idea of Deckard being a replicant. I am trying very hard to allow myself to accept that this interpretation is fine and can coexist with my preferred version of events. Needless to say, I find it extremely difficult. But in the spirit of fairness and with the cruel onslaught of empirical reality mocking my every cherished belief in the matter, I must now concede there is pretty much absolute, unambiguous, undeniable confirmation that if you watch the versions of the film where Deckard has the unicorn dream/reverie while sitting at the piano, the scene in question stands as irrefutable proof that Deckard is a replicant. Ridley Scott said as much in a 1982 interview with Starburst magazine in an article entitled The Blade Cuts:
Ridley Scott: Did you see the version [of the script] with the unicorn?
Alan McKenzie: No.
S: I think the idea of the unicorn was a terrific idea.
M: The obvious inference is that Deckard is a replicant himself.
S: Sure. To me it's entirely logical, particularly when you are doing a film noir, you may as well go right through with that theme, and the central character could in fact be what he is chasing.
M: Did you actually shoot the sequence in the glade with the unicorn?
S: Absolutely. It was cut into the picture, and I think it worked wonderfully. Deckard was sitting, playing the piano rather badly because he was drunk, and there's a moment where he gets absorbed and goes off a little at a tangent and we went into the shot of the unicorn plunging out of the forest. It's not subliminal, but it's a brief shot. Cut back to Deckard and there's absolutely no reaction to that, and he just carries on with the scene. That's where the whole idea of the character of Gaff with his origami figures — the chicken and the little stick-figure man, so the origami figure of the unicorn tells you that Gaff has been there. One of the layers of the film has been talking about private thoughts and memories, so how would Gaff have known that a private thought of Deckard was of a unicorn? That's why Deckard shook his head like that [referring to Deckard nodding his head after picking up the paper unicorn] (p.29)