What many others have said:
Unicorns, possibly. That's not too much of a variation from the horse, with all features within the mammal family. All it would have taken was a horse with a somewhat pointy forehead, and from there a nub, from there a point, and then to a horn.
Dragons and gryphons? No way. Amongst all the other problems, those models of wings are limbs. That means they'd be six-limbed vertebrates.
The only way to get there would be a complete reconfiguration of evolution. Or, to put it another way, the only way to have them occur is if we didn't. I'm calling no go on gryphons and dragons.
Edit: My girlfriend has persuaded me to rethink my position on dragons. Something that could be recognisably called a dragon may have been able to evolve. I'm willing to believe that. It wouldn't exactly like our conceptions of dragons, if nothing else it'd have been much, much smaller. But it would be dragonlike.