My headcanon is that Matter-horn was an extremely bright earth pony scientist that was working on an invention that would have allowed earth ponies and pegasi to use artificial unicorn magic.
 
After her assistant and best friend was nearly killed testing the device, she abandoned it, declaring the artificial horn too dangerous for ponies to use.
 
However, one of Maretropolis’ crime syndicates caught wind of the device’s existence, and proceeded to kidnap her friend from his hospital bed. They intended to force him to use his photographic memory to mass produce the device.
 
Not finding him in the hospital, she overhears the police mentioning that members of one of the syndicates, (The Coif Colts) had been spotted outside the hospital before the estimated time the abduction took place. They know where the hideout is, but don’t think they have the horsepower to handle the entire gang.
 
She returns to her research lab and takes the artificial horn out of storage. To her surprise, the device isn’t lethal to her. She makes some quick modifications to it to give the horn various elemental abilities to help her in combat, and puts on a hazard suit and goggles to avoid being ID’d.
 
“Nopony kidnaps my friend and gets away with it.”

So I take it this is what the “real” Masked Matter-Horn looks like?