Talk:Cast/@comment-5288784-20170211153513/@comment-5837525-20170211155053

They appear to be announced primarily through Twitter by the actors themselves, so you could try Peña's page (@realmichaelpena), the official MLP movie Twitter (@MLPMovie), and for good measure, Meghan McCarthy (@MMeghanMcCarthy) and Michael Vogel's pages (@mktoon) as they usually retweet each announcement.