I thought they couldn't because of selling most of them
From what I've heard, they lost "copyrights" to most or all of the '80s characters aside from Applejack and Spike. However, I am not sure how this all actually goes and how copyrights affect anything here.
Plus, since Minty is being sold again perhaps they got copyrights back or something, so I don't see why they couldn't use her. She seems to be the only character from the past generations that has re-appeared.