I recognize that Hasbro has made a team of non-Mane-6 Power Ponies, but given that they pretty much copied the origin stories of the superheroes they’re based on, I have elected to ignore it. So I made my own.
 
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