<Snipped quote by Windicator>
Long answer: This goes back to the early days of BIONICLE when LEGO was grabbing a bunch of words and grammar from the maori language, and in maori, like other languages with polynesian roots, does not pluralise by adding an s or anything else to the end of the noun.... So, one matoran, two matoran, many matoran.
Thanks, I've just heard it said in many ways an I was unable to really recall any official material on the spot.