Well, we can say that we have also adopted that word in spanish, so even you don't need a translation. We say that someone is a "freak" or "freaky" (we pronounce it: "friki") when that person has an oddball behavior or, in other words: a weirdo :P Another synonyms or common expressions for this meaning: "raro", "bicho raro", "perro verde", "fan de Star Trek/Wars/The Big Band Theory... *poke to Froz'n

I'd say "persona extravagante", "estrambótico", "chalado", "estrafalario" or "tocado de la cabeza".

Fenomeno

If you mean "freak" as a rare guy , we also use the word "freak" when we talk about a rare guy. "Eres un freak" "you are a freak".

Another exprenssions: exòtico, anormal, extraño, monstruo, or monstruoso, tarado, adefesio