The first movie released is the original. A movie released later which continues the story is a sequel. A movie released later which pre-dates the original story has been given the term prequel. Pre-sequel is redundant. It implies that either:
a) a sequel could contain a story set prior to that of the original, which it probably could until prequel was coined; or
2) a prequel could be released before the original, which is patently nonsense as this would make it the original, the original a sequel, and whoever supported the notion, an idiot.