Apparently Assyrians used to hunt wild oxen similar to the buffalo and the animal was often depicted in drawings. The animal was often shown in profile in their drawings so you could only see one horn. This lead to a common name for the animal, "one-horn," like we use the phrase, "long horns," for certain types of cattle. The original intent was not that the animal only had one horn but this was confused in the Kings James version.
When I first came across this silly fact, I thought it had to be a blatant error by the people translating the King James Bible. There is no such thing as unicorns!! However, this is not entirely true. It's the English language that has changed since the translation. The word "unicorn" now has an entirely different meaning than it did before. Today it means "a mythological creature with a spiral horn on it's forehead." That was not the definition at the time of translation. The Hebrew word that is used that is translated into unicorn means "a horned animal." That's more of a general description for several different animals. What did unicorn mean back then? If you look at the word unicorn, you'll find something very interesting in the root word. It originated nearly 800 years ago to mean "one horn." If you look at the Latin or scientific name of an Indian Rhino, it's "Rhinoceros unicornis" So it's very probable that the translators and English back then meant either a one-horned animal like the Hebrew word suggests, or a rhino.