Why is it a unicorn instead of an unicorn. Unicorn starts with a vowel and so shouldn't it be an?
The article(a/an) that precedes a word is largely dependent on the way the first syllable of that word is pronounced, though many people follow the rule that words beginning with vowels must be preceded by 'an' and words beginning with consonants must be preceded by 'a'.
Here, unicorn begins with the vowel 'u' but it's pronounced more or less like 'yoo'. 'Unicorn' begins with a consonant sound, so we use 'a' before it.
Some other examples are: a user, an honour, a university, a European.
When "u" makes the same sound as the "y" in "you," or "o" makes the same sound as "w" in "won," then a is used. The word-initial "y" sound ("unicorn") is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring "a."
a union, a united front, a unicorn, a used napkin, a U.S. ship, a one-legged man.