When did people start calling "manga" as "mangos"?
It has nothing to do with the fruit. They are derogatory versions of their associated words to make fun of the way they are mispronounced in English (with the "a" vs "ah" sound). Unfortunately, I don't have any references. Lurkmoar has a page dedicated to the anime/manga board on 4chan.

EDIT: Just to clarify, I think people started using the "animu" and "mango" terms on /a/