grammaticality - Was "an unicorn" ever correct?

According to Biblehub and Bible Gateway, King James's Numbers 23:22 says:

God brought them out of Egypt; he hath as it were the strength of an unicorn.

I don't have a hard copy to check. Is that simply a copied typo?

Answer

No, it's not a typo. Words starting with u started with a diphthong until the 18th century. This was part of the Great Vowel Shift. The vowel started changing from /yː/ (its original vowel in French¹) in Middle English, migrated through several diphthongs, and ended up at /juː/ sometime around the 18th century.

See Ngram.

¹ At least the upper classes, who were descended from French-speaking Normans, used the original French vowel /y/ for French words spelled with 'u' in Early Middle English. I don't know whether there's any evidence for how the lower classes pronounced this.