Even though unicorn starts with a vowel it's pronounced like "yoo" which sounds like a consonant. Similarly it's the same with the word - user.
( not an user but a user )
I know it might only be for this one word, bit why do we say 'a unicorn' instead of 'an unicorn'?
Use A before words such as "European" or "university" which sound like they start with a consonant even if the first letter is a vowel.
Also use A before letters and numbers which sound like they begin with a consonant, such as "U", "J", "1" or "9". Remember, it is the sound not the spelling which is important.
Internet---A vs an
Because a vowel is a sound, not a letter. As kids we're taught that vowels are letters, and that's the simplest way to learn it, but a "vowel" actually describes a type of sound, one that is syllabic and not produced with some constriction of the air flow. Thus the letter U in English often at the beginning of a word produces a combination of a consonant (semivowel) and a vowel, that is the combination /ju/ (in IPA), same goes for a word like "eugenics". The /j/ sound (i.e. The "y" sound in "yarn") is a consonant, thus "a" is used rather than "an".
Interestingly enough, /h/ is considered such a mild consonant sound that in some dialects of English (particularly British ones), it's proper to use "an" before words like "historic", despite their beginning with a consonantal sound.