Alrighty folks!
In my definition under "Unicorn" in the DADoES/Blade Runner Glossary it reads the following:
Unicorn
Movie/Game
Movie
A. The daydream sequence Deckard has as he is sitting at the piano is of a unicorn galloping in slow motion. The scene is only in the Director's Cut.
B. The unicorn is the last of three origami figures that Gaff crafted throughout the movie. It appears at the end of the film when Deckard leaves his apartment with Rachael. She knocks over an origami unicorn as she walks towards the elevator. Deckard sees it in the corner of his eye, and picks it up nodding his head.
One interpretation is that the unicorn was simply a message to Deckard to say he knows he's with Rachael and will let her live. Another interpretation is that the unicorn is Gaff's challenge to hunt them both down.
An interesting article regarding this theme can be found here.
http://scribble.com/uwi/br/brfaq/unicorn.html
FYI:
Sebastian's apartment is full of bastardized creatures, part man, part machine, and part animal. There is a stuffed unicorn on Sebastian's work table (screen right, as the mice scurry over scattered paraphernalia while Sebastian sleeps).
Speculative question:
When Deckard dreams of the unicorn it should be a private dream, yet Gaff, by leaving an origami unicorn on the landing hints that he knows about the dream. How could this be unless Gaff has seen the data implanted in Deckard's mind?
Game
Gaff leaves his infamous calling card for Ray in the game. Depending on your gameplay, Ray will either find the origami on the Moonbus floor, or on the ground outside the Moonbus.
DADoES and Blade Runner Glossary
Also...
Unicorn at Wikipedia