Earlier today it was confirmed that Tony-Award winning musical actress Lena Hall lent her voice to My Little Pony. She tweeted this image of herself in the recording studio, claiming it was being used in Season 5. This isn't the first time Hall has been linked to ponies.
Earlier in 2014 following her Tony-award acceptance for Best Actress in a Musical, she listed My Little Pony as one of her biggest influences, alongside the likes of Marlon Brando. It's not yet known in what capacity her voice is being used, but given her musical background and Jayson Thiessen's tweets it's not outlandish to assume she will have a singing role.
It is confirmed that the episode was written by Amy Keating Rogers however.