"The Unicorn is a Mythical Beast." —James Thurber, The Unicorn in the Garden
Two things are true about the Noah's Ark story. First, there were two flooding catastrophes in slightly prehistoric times in the Mideast. The Black Sea was perfectly dry one month and flooded the next month. Also, some type of tsunami caused either by a volcanic explosion on an island in the Mediterranian or a meteor took place.
Second, we know that if we don't build a huge zoo, we're going to have a mass extinction on earth in this generation.
Unless Noah's veterinarian had a huge stock of tranquilizers, with 55 dinosaurs and 8,000 other species on board, its a safe bet that the last unicorns and at least a few other species were eaten during the voyage.