In the Blade Runner Director's Cut a dream and a miniature of a unicorn are present. These are interesting images considering the tone of the movie is dark and violent. The contrast between the tone of the movie and the imagery of the unicorn is important because it's the director's attempt at clueing us in.
The fact that the two proceeding miniatures (the chicken and the matchstick man) before the one of the unicorn have meanings that are easy to guess move viewers to speculate if the unicorn is an artifact of symbolism. What the unicorn is representing may not be so easy. The chicken, the first miniature of the movie, is made in the office of Deckard's boss. When Deckard tries to refuse the job of hunting down a new batch of replicants, the chicken is used to poke fun at Deckard's reluctance. The matchstick figure of a man is crafted when Deckard's relationship with Rachel and the violence heats up. Like I mentioned before, the meaning of these miniatures are straight forward, but they are important because the director probably did this purposely to draw viewers' attention to what the unicorn may or may not mean.
What does the unicorn symbolize? The first clue is the dream Deckard has while sleeping at the piano. His dream is of a vital, white unicorn running through a green, lush landscape. Deckard is probably dreaming of what he wants his life to be like. Deckard wants freedom from his job, his dark, oppressive surrounding, and, most importantly, from his impure, meaningless life. This is why the unicorn is white (purity) and why the landscape is lush (lively and rich of meaning). At the time Deckard dreams of his freedom, he doesn't know how it can be achieved. That is where Rachel comes in literally and figuratively. She, through Deckard's compassion and sexual interest towards her, starts to help Deckard see the way.
The next time the viewers see the unicorn is at the end of the movie when Deckard and Rachel is fleeing from his apartment. In the hallway, on the ground, is a miniature unicorn constructed from an empty bubblegum wrapper. It's very important that it is made of a gum wrapper---one side paper, the other side tinfoil---because it introduces three possible interpretations. First, the unicorn is simply referring back to his dream. It is a sign to both Deckard and the viewer that his desire of freedom and happiness is actually going to happen. He and Rachel are going to travel together in freedom with whatever time Rachel still has to live. It is a very "happily ever after" ending for a movie so grim, but it is possible due to the graceful images in his dream. Second, the unicorn made from a gum wrapper may also be reminding viewers who holds Deckard's "unicorn". The wrapper is paper on one side to represent the humanity of Rachel, and the tinfoil on the other side to represent the fact that she was created as a replica. Third, and probably the most controversial, the unicorn may be hinting to the viewers that Deckard is a replica. In this case, the tinfoil is representing the fake, replica side of Deckard, the man who has his dream of the "unicorn". In the movie, there is evidence that the director is using imagery a couple times throughout the movie to convey who someone really is. The first miniature was of a chicken to show that Deckard was acting cowardly in some people's eyes. The snake in the dressing room hinted that Zhora was a replica (remember the tattoo?). The third would then be the unicorn symbolizing Deckard as a replica.
It is quite possible that all the interpretations suggested in this post is right (or, to be cynical, wrong). Why would Ridley Scott stop at only one possible meaning of the unicorn? He wouldn't. All well respected and important works of fiction have multiple meanings, and Blade Runner has endured the test of time far better than a lot of science fiction movies in the past. Heck, how many people even bothered watching Mission to Mars that came out less than a decade ago? Enough said.
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