"The mythical unicorn belongs to the land of Never Was, but the people of long ago didn't simply believe in the beast on faith. Historians think that the Vikings brought narwhal tusks back from their sea journeys. Traders peddled the tusk as a unicorn horn, believed to be an antidote to poison, and the wealthy fell for the ploy. Of course, no one had ever seen a unicorn. But it was mentioned in the Bible, so it would be heretical to say it didn't exist. And at one time, there was a belief that land animals had marine counterparts, so even if a tusked whale were found, it wouldn't disprove the existence of the unicorn."
Above quote taken from this link here
Pre king James bibles must have translated the relavant bible verses as Unicorns as well (as obviously Vikings were pre King James)
Verses that the King James translates as Unicorn the NIV opts for wild ox.....