Why does everyone think the unicorn represents Deckard being a replicant?

Love this movie. Have only ever seen the Final Cut (multiple times). It never occured to me, until I recently read it somewhere else, that the unicorn might represent Deckard being a replicant. I had always assumed the unicorn represented Rachel or Deckard's grappling with his feelings toward her (and more broadly, feelings toward something that isn't human and which is supposed to be hunted). When she steps on the unicorn origiami as she leaves Deckard's apartment, I assumed it was an omen or a warning from Gaff that she'd be next.

So I guess I'm wondering why everyone assumes unicorn = replicant.