That's not really a fair comparison. We know what unicorns are. If someone discovered an animal that looked like a horse but had a horn growing in the middle of its forehead, they could call that a unicorn and most people would agree with them. It's theoretically possible (albeit extremely unlikely) that that animal exists somewhere.
With God, on the other hand, the definitions are constantly changing and self-contradictory. Even as a theoretical concept God has never been described with any degree of coherence, so the idea of whether or not it "exists" is moot. Something needs to exist as a concept before the question of whether or not it exists in the real world can even be asked.