"Wait!" you say. "I don't want it to be obvious. I want it to come as a surprise."
Well of course you do, but you can't blindside the reader. Throughout the book you are building a relationship with the reader. You earn her trust, and if you throw it away by pulling something - say a unicorn - out of left field, you'll lose that relationship.
So how do you do it? How do you put a unicorn in the corner, but not let the reader realize they've seen it until the right moment?
The answer is - be a magician. Remember my post on filtering through character? If not you should check it out. This is yet another example of how that can help your manuscript. If your MC doesn't pay attention to the unicorn even though you've made it clear he's there (by say putting glittering hoofprints on the floor in an earlier scene), then chances are your readers will ignore it too.
What? Glittering hoofprints and the MC didn't notice? Maybe she was too distracted by the dragon at the time. The dragon who we find out in another scene has talons, not hoofs. Yes, the reader MAY still see what's happening. But if you are careful enough with your slight of hand/distraction techniques, they may be more invested in whether your MC will figure it out on time. And at least to this writer that's a better alternative than breaking the reader's trust.