Re'em are mythical(ish) animals mentioned in the Bible and Jewish folklore. I say mythical-ish because today it is pretty widely agreed that Re'em are actually exaggerated versions of either Aurochs or Arabian Oryx (though some creationists think it anything from Rhinos to triceratops). Re'em are responsible for one of the funnier Biblical translation goofs, as the King James Bible translated Re'em (which was accepted as some kind of Ox-creature) into freaking unicorn.
Re'em themselves are some kind of giant ox. A few of the stories floating around them include:
After the biblical flood, there were two Re'em left on either side of the world, east and west. They ignore each other for seventy years until mating season. Then they canoodle and the female kills the male. The female's gestation period is twelve years, after which she will birth twins, a male and female. These two immediately wander off to opposite sides of the planet and start it again like weird bovine Phoenixes.
When Noah was filling up the ark, he had a few Re'ems join him. One version says he couldn't fit them into the ark, so they were either strapped onto the sides of the boat, or they were big enough that they could just swim around after the boat. Another version says that the flood happened just as the baby Re'ems were born, so they could fit after all.
When David was still a shepherd, he climbed on a sleeping Re'em's horn, thinking it was a mountain. Then the Re'em lifted up its head and David realized he'd done goofed. He prayed to God to save him, saying that he'd build God a temple, and God sent a lion. The Re'em bowed to the lion and David got off while it was down. Then God made a deer run by so the lion would go chase that instead of eating David.