If you want to claim unicorns are in existence currently, you'll have to explain why no one has seen one and why we've never found unicorn carcasses, etc. If they existed in the past, then there should be something to support that hypothesis in the fossil record. As far as I know, there isn't, so if you make that claim, you'll have to explain that as well. If you claim something to be true, then the burden of proof is on you.

@dawn: Wait, your biology teacher used that argument? How can someone who lacks a very basic understanding of evolution be teaching biology? I'm sure you already know why that argument is flawed. The evolution of ancient primates into humans took millions of years. It would not be possible to see other primates evolve into humans in a lifetime or probably even a millennium. Chimpanzees, apes, orangutans, etc. still exist today because evolution caused a new species to form in addition to (not in place of) the existing one. It's called speciation, and since all living things evolved from one common ancestor, as you can imagine, it has happened a lot of times! But remember, the earth is very, very old, so major speciation events are not necessarily as common as I make them sound. But all living things are still evolving. We're not necessarily forming new species, but the genetic makeup of our populations is constantly shifting as we pass on our genes from one generation to the next.