Hmmm.... what is a re'em? Like God, I mean wikipedia gives the answer:
No it doesn't. It says (paraphrased): "We have no idea, it might have been based on some real animal, or real animals names may be cognates of it." However, reading a bit more
In Jewish folklore, the Re'em was so large that Noah had to strap it on the side of the Ark, and King David, while still a shepherd, mistook its horn for a mountain and climbed it, then the Re'em got up and frightened David.
Doesn't exactly sound like a real animal.