Butters was originally a background character in the school; being around since the very first episode, "Cartman Gets an Anal Probe" (meaning he is one of the few to have been originally made of construction paper).
Eleven episodes later, Butters opened his mouth for the first time in "Damien", when Damien goes around burning the playground equipment; he screams. And he sounds like a girl.
One season later, in the episode "Roger Ebert Should Lay Off the Fatty Foods" he speaks for the first time; with the words "me too", and he recieves another speaking role in the following episode "Clubhouses". They're only two short lines, and still don't sound like his modern voice. Also, before this, he is called "Swanson" in the episode "Conjoined Fetus Lady".
Another season later, Butters is heard going "Oooh" while touching Cartman in South Park: Bigger, Longer & Uncut.
Cut to literally a few weeks after the movie came out, it's the episode "Two Guys Naked in a Hot Tub", and Butters is finally introduced properly and formally.
That is all the history behind the slow revelation of the character.