Ok, After studying this plenty and talking to plenty of English teachers and classmates, I've got a few thoughts on this.
The accepted interpretation of this dream is that Deckard is a replicant, which is backed up by the origami unicorn at the end of the film - Gaff read his 'file' and knew that he had the Unicorn implanted into his head
However, he is NOT the '6th replicant' that escaped from the shuttle
In an interview with Scott during a making of, he revealed that originally there was to be another replicant fromt he craft, a female replicant called 'Mary' who was to be killed shortly after Priss, shot through the door of a wardrobe by Deckard.
However, Scott thought that her addition dragged the movie out too much and decided to remove her from the script. However, soem scenes had already been shot, including the scene where it is said that there were 'Six Replicants', and this plot hole wasn't sealed properly
That said, In the interview he also SAID straight out that Deckard was a replicant, and as he's the director, he would know (IMO). However, Harrison Ford has in interviews alluded to the fatc that he played deckard as a 'Human' ratehr than a replicant, and sees Deckard as a human.
Onto the Unicorn - If Deckard IS a human, or perhaps even if he is a replicant, the Unicorn can be seen as an allusion to Man's desire for nature and the wild, somethign which has been destroyed by Urbanization but soem humans may still remember
So yeah... my 2 Cents on this issue