Unicorn of the Sea

The aquarium (yes, I love fish) has a sign in their new shark exhibit. Something to the effect of "Sharks are the unicorns of the modern era". But how can that possibly be true? Sharks are real and unicorns weren't, right? Or at least that's what I always believed. But the people running the aquarium know more than I do about many things, and sharks are (hopefully) one of them, so maybe it could be true.

Were unicorns real creatures at some point? Did the roam the land the way the sharks roam the sea, being drawn to blood, ferocious animals that everyone feared? Were unicorns seen as the most vicious of animals, hunted to extinction to protect families from the constantly looming threat of being gored when they least expected it? Did unicorns stalk their prey, the tip of their horn the only thing visible, warning them of the doom at hand? Did merchants sell unicorn teeth to tourists visiting the forest for the first time?

Mayhaps sharks are really just a myth. Maybe there's some benign creature out there, something that afar appears to be this deadly evil. Perhaps noone has ever died because of a shark, all of the stories have been made up, imagined, designed to keep children from running out and playing in the ocean while their parents are safely sleeping. It's possible that all of the pictures were faked, all of the stories fictitious, all of the fear created and not due to something that's real. Plausibly sharks will be the creatures of myths years after they're discovered to just be a variety of tuna, their shape and size appearing changed by the refraction of water.

Truly, though, the quote is supposed to be taken more literally. There used to be no sharks and many unicorns, and over time the unicorns changed. Being hunted out of the forests and off the plains, the only place left, the only place safe was in the water. Through the years, the unicorn lost its horse-like appearance. Being a magical, mystical creature, of course, it can do that. As time passed, the unicorns became sharks.

How many stories about sharks /were/ there when unicorns roamed the earth?