Fulano is a colloquial term for "so-and-so" or "what's-his-name". It's a placeholder used to refer to a man whose name you don't know, or more fittingly in this song, a name you don't care to use.
In "El Perdedor", the singer spits out the word with contempt, referring to his rival as just some random guy: "Fulano con el premio y yo el perdedor" (What's-his-name with the prize, and I'm the loser). It’s a powerful, dismissive word that perfectly captures the jealousy and bitterness of losing someone you love.
We have all heard the old rule that you don't know what you have until it is gone, and bachata legends Aventura turn that exact proverb into a dramatic, jealous meltdown in "El Perdedor". Romeo Santos starts out admitting he watched his partner cry and let her life sink into boredom while he just sat there listening without really understanding her. Another guy stepped in, stole his "tesoro de duende" (goblin's hidden treasure), and treated her right. Even though the singer bluntly confesses "el culpable he sido yo" (I was the guilty one), he cannot stop hating the new man for winning the battle for her love.
Instead of taking the loss gracefully, he sends a petty message straight to his rival. He wants the guy to know that he stays awake tossing and turning at night, tormenting himself by picturing them together. He calls himself an envious, childish loser for assuming he was irreplaceable. Look out for the Dominican slang word "Fulano" in the final lines, which works like saying "some random guy" or "Joe Blow" in English. He ends the track watching this absolute nobody walk away with the prize while he is left holding the title of the loser.