I recently almost had one of PMV's muted (the first one I did with non-pony music), and know of people who have gotten muted so I would like to post how to get out of the red:
Copyright Disclaimer Under Section 107 of the Copyright Act 1976, allowance is made for "fair use" for purposes such as criticism, comment, news reporting, teaching, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. Non-profit, educational or personal use tips the balance in favor of fair use.
American^, but most music is controlled by American based corporations (ex. Warner Brothers) so this can often save you if posted in the description of a video. (As long as your not trying to monetize on the video, its non-profit, if you get compensation from Patreon per piece, then I'm not sure if this will work for you)
However a full-proof method is to tag all your PMV's with copyrighted music as Comedy, or Satire, stating that the video is simultaneously a parody of My Little Pony, and of the song you are using.
If you aren't monetizing your videos this will almost always work. I haven't had much trouble with copyright as many of my PMV's are of music from within the fandom, however these tricks have worked for friends.