Here's the main voice cast for Monster Mash-Up Musical

THE EQUESTRIA GIRLS:
Tara Strong as Twilight Sparkle
Cathy Weseluck as Spike
Rebecca Shoichet as Sunset Shimmer
Andrea Libman as Fluttershy
Shannon Chan-Kent as Pinkie Pie
Tabitha St-Germain as Rarity
Ashleigh Ball as Rainbow Dash
Grey DeLisle Griffin as Applejack
Kelly Sheridan as Starlight Glimmer
Kathleen Barr as Trixie Lulamoon

THE GRIMWOOD GIRLS:
Chloë Grace Moretz as Sibella Dracula
Kari Wahlgren as Phantasma
Kyla Rae Kowalewski as Tanis
Nika Futterman as Winnie Werewolf
Vanessa Marshall as Elsa Frankenteen
Roger Craig Smith as Matches
Ashly Burch as Enid Meetle
Kate Micucci as Galactica
Hynden Walch as Lagoona Black
Myrna Velasco as Minimal

SPECIAL GUEST STARS:
Justin Timberlake as Henry, a motorcycle daredevil human.
Selena Gomez as Zoe, a young world-traveling adventuring sorceress and leader of the Wild Cats.
Julia Roberts as Tahlia, a tough but kind talking tiger who is a member of the Wild Cats, and acts as a mother to Zoe.
Dwayne Johnson as Leonard, an intimidating but big hearted talking lion who is a member of the Wild Cats, who acts as a father to Zoe.
Tom Holland as Jason, a clumsy and feisty talking jaguar who means well and is a member of the Wild Cats, and occasionally gets into a rivalry with Zoe, as if they were a competing brother and sister.
Will Ferrell as George Sullivan, a corrupted human accountant who deeply hates the Rainbooms because of having been dumped by his girlfriend one summer for being their biggest fan at the time, effectively eliminating his shot of becoming a cop.
Benedict Cumberbatch as Scarface, the leader of the demons and succubi who wants to take over all of Monsterdom.
Kristen Schaal as Sarah Scientist, a human scientist and inventor who is a great friend of the Grimwood Girls and provides them with gadgets, weapons, and inventions for their adventure.
Wallace Shawn as Mr. Mysterio, Zoe's old magic teacher from high school who eventually retired.
Kevin Hart as Slither, a talking snake who is eventually forced to come with Sullivan on his goal to get revenge on the Rainbooms.
Alec Baldwin as General Grant, a general officer who is in charge of a top secret weapons facility that the Rainbooms and Grimwood Girls visit as the facility was one of Scarface's targets.
Wendie Malick as Jade, a glowing purple banshee who guards the Crystal Cave (one of the Rainbooms and Grimwood Girls' locations on the way to MacWright's treasure).
Robert Downey Jr. as Igor, Scarface's hunchback assistant who slowly becomes increasingly sick and tired of Scarface mistreating and discouraging him.