Here are the characters’ names, from left to right:
Saddle Rager - Feather Fluff
Masked Matterhorn - Galaxy Star (based on G1 Galaxy)
Mistress Marevelous - Buttercup Cuddlemuffin
Zapp - Sky Song (uses Electronica as a stage name)
Radiance - Azure Glory (based on G1 Glory)
Fili-Second - Deft Comet
Their origins (in a very brief summary):
Masked Matterhorn: learned elemental magic
Fili-Second: exposed to radiation (originally from a nuclear explosion, but that seemed a little far-fetched)
Zapp: received amulet as a gift
Radiance: enchanted her own jewelry
Mistress Marevelous: inherited a magic lasso from deceased father
Saddle Rager: prank backfire
Humdrum: distracted a villain during a fight on the way to a convention
Susanne Blakeslee as Azure Glory
Hynden Walch as Clever Comet
Tabitha st. Germain as Buttercup
Jennifer Hale as Galaxy Star
Janice Kawaye as Feather Fluff
Debi Derryberry as Stitches
Buttercup/Mistress Marevelous
Sky Song/Zapp
Galaxy Star/Masked Matterhorn
Clever Comet/Fili-Second
Azure Glory/Radiance
Feather Fluff/Saddle Rager
Fluttershy/Captain Equestria
Rainbow Dash/Iron Dash
Pinkie Thor
Applejack/Hawkeye
Rarity/Black Widow
Twilight Sparkle/Hulklight
Spike/Spider-Dragon
Shining Armor/War Machine
Octavia Melody/Doctor Octopus
Bon Bon/Sandmare
Vinyl Scratch/Electro
Lyra Heartstrings/Rhino
Cloudchaser/Vulture
Flitter/Venom
Discord/Loki
Sunset Shimmer/Enchantress
Prince Blueblood/Baron Zemo
Trixie/Taskmaster
Flim & Flam/Red Skull (equally)
Suri Polomare/Crimson Dynamo
Gilda/Abomination
Starlight Glimmer/Deadpool
Lightning Dust/Green Goblin
Princess Luna/Nick Fury
Princess Celestia/Daybreaker
Mane-iac
King Sombra/Doctor Doom
High Heel
Pharaoh Phetlock
Long Face
Shadowmane
Smudge
Avarice
Flash Sentry/Ghost Rider
Zecora/Black Panther
Doctor Whooves/The Lizard
The Destroyer
Queen Chrysalis/Veranke
J.A.R.V.I.S.
Spitfire/Phil Coulson
Tempest Shadow/Punisher
Lord Tirek/Thanos
Galactus
Citizens of New Ponyville