 TOHO had planned to release this movie as "Godzilla VS Destroyer" to  the English language markets. But when they realized that  copyrighting an everyday word like "Destroyer" for their monster  would be difficult, they released the film as "Godzilla VS  Destoroyah" since Destoroyah is close to the English version of the  word, and was a unique name that could be copyrighted easily. When  TOHO had the movie dubbed they made sure the monster was called  "Destroyer" on film since that was the intended name.