For Pony AU, since I’ve twisted my Headcanon so far the show can’t hold it.  
Name: Gear Grinder (formally Brass Banner she changed it when she got her cutie mark)  
Race: Unicorn  
Sex: Mare  
Sexuality: Lesbian  
Age: Young adult  
Height: 19 hands (193.04) huge compared to most ponies  
Weight: 920kg (2028.25lb)  
Job: General Practitioner Specializing in Prosthesis (Magic and analog)  
Hobbies: Working out, building odd mechanical device, Reading, traveling  
Likes: Exercising, Math, Jim, martial arts, trains, mechanical device, cooking.  
Dislikes: magic, most of her Family (she likes her brothers and dad), Celery (she has a phobia of it), crowds
 
Magic:
 
transformation however Gears magic is so unstable that she can only do light Levitation (up to 20kg) and magic transfers safely(controlling mechanical devices she put magic into) . Anything else will most likely to go out of control, in minor cases this will result in things turning in to oranges ( or other objects) only to turn back in a few minutes. In serious cases changing an object in to an explosive that renders the area into white hot flames. Sometimes she will wake up from a nightmare only to find her magic has turned her it to a chicken or the like in the night, she turns back by the afternoon usually so usually she just ends up grumpy.  
Unfortunately for her she can’t stop using her magic due to her magic stores are also too high for her body. If she don’t use or bleed off the magic it will start burning and melting her organs, until her death that’d come in two to three days. She carries an epipen like device that will burn off her magic if she starts having an attack, it makes her sick and hungry after using it but it’s better than melting organs. Given her strict training on magic use and the magic drain of her job she’s only had two such attacks. Her training also has made those times she does lose control safer for those around her, more oranges and less exploding.
 
Background:
 
Gear was born in Cantorlot to the Reems a family of unicorns famous for the magical powerful ponies that run in the family. However Gear was born with a horn that was too short to properly control anything besides simple spells like levitation, normal this on its own is a fair handy cap for a unicorn. But Gear inherited also the huge magic stores her family is famous for, making for very unstable magic. After She went out of control turning her preschool class into dragons. She was placed in to a special section of Celestias’(may make an oc Alicorn ruler for this AU?) school for gifted unicorns, one that deals with dangerously gifted ponies. There she learned how to control and use her magic, as well as meet her mentor Gizmo. Gizmo seeing Gears talent for building mechanical device something Gear did to help her deal with her overbearing magic. Gizmo to her under his wing, teaching Gear the skills need to be a prosthesis technician, being a powerful unicorn himself he helped Gear though her magic unstably. Much of this training was spent traveling do to Gizmo’s job has him travel to the most remote, areas of Equestria and other countries to build fake limbs for those that can’t get to the few normal specialists; as well as odd cases like dragons or giant spiders. Gear loved the work and formed a strong bond with Gizmo. Trouble came however when her grandmother decided that Gear since she wasn’t exploding things anymore, that she was “cured” of her disability and like a proper Reem and preform the traditional Reem magic exam. This didn’t end well for anyone, Gear overreaching and stressed went completely out of control causing a huge explosion that injured 15 ponies, and the damage would have been worse have there not been her mother and her brothers who blocked most of the runaway magic. She runaway from Canterlot cutting ties with her family and few friends there, throwing herself in to her prosthesis training. Within a few years becoming the second youngest pony to become full technician; taking control of the disuse Prosthetic clinic in Miller’s bit even though it was closer to Canterlot then she’d like. She’s recently started reconnecting with her family, learning that the family she care for her didn’t hate her for the accident something that she’d always feared.
 
Personality:
 
Gear tends to intimidate ponies she just meets given her large build and default expression being read as anger. Well Gear for the most part finds this useful at keeping ponies she don’t care about from wasting her time, She dislikes that she scarce her first time patients. She’s horrible at reading the emotions of other ponies if she forces herself she can usually get around this by using tricks Gizmo teach her but it leaves her drained. Gear tends to stay away from crowds, since from her point of view that energy could be used for ponies she cares for instead of for small talk with ponies she don’t. With her friends she tends to be blunt but laid back, becoming pretty friendly once she get to know others. She’s fiercely loyal and protective of her few friends, helping them with their problems even if it is hard on her. likewise if she decides she don’t like somepony, it’s near impossible to get her to change her mind.  
Well Gear sucks at dealing with emotional pain there are few ponies that are as good as her at picking up on physical pain. Gears very good at picking out ailments and treating them, though unlike most unicorns she can’t use magic to help; using instead the first aid kit she carries when she goes out. However her strong desire to heal others sometimes leads to problems when ponies don’t want help, since Gear doesn’t tend to take no as an answer easily; usually she’s scare the pony into letting her help. For her friends she may physical force them to let her help, scared to lose them. Since becoming friends with Steam this has gotten less common, both since Steam is helping her deal with her social problems (Gizmo wasn’t the best teacher with this since he had trouble figuring out what Gear was having trouble with and was not very social himself.) also well as being able to talk the pony in need of help, into getting help.  
Gears very routine based, and if something don’t allow her to do her routine she ends up grumpy; thankfully most of them are in the early hours morning so it’s rare that she is bugged. Most of these routines are things she uses to control herself such as working out, practice fighting patterns and magic use.
 
Jim: Race: Dog turned into a magic crystal (the same used to power prosthesis)  
Sex: male  
Age: older than Gear
 
Jim is Gears pet and friend, helping her in the shop be fetching things, powering her weirder creations and acting as a mobile magic transport for important mail and packages to come though. He was her dog until Gear had an accident her magic turning into him a magic crystal, since this crystals “eat” up magic used on them there was no why to turn him back. Since he was still alive this new form didn’t bug her much, she made a body for him and uses her magic to “feed” him. It’s rare to see Gear go anyway without Jim with her, usually sitting on her head.
 
Trivia:  
Gear’s phobia of celery started after her Grandfather think that the young filly was too fat, forcing her on to an almost all celery diet, which ended up with a very sick Gear, Her mother quickly noticed something was wrong and get her away from her Grandfather but the damage was done. usually she’s fine with passing celery in stores but if you waved it in her faces she’d panic, twos tons of fighting train pony is not something that you want panicking so Gear is trying to get over the fear.  
Gear learned how to fight from her dad (who is an officer in the Guard) and later Gizmo; she’s very good at it and is faster than her size would make you think.  
Though her running speed is quite slow, she can pull a lot of weight over a long distance. Her traveling limit being around 5 tons, with her top pulling limit being around 15 tons. Though a earth pony of the same build would be able to move more the her, Gears about as physical strong as a unicorn can get.  
She likes doing taxes, given the only accountant in Miller’s bit is a bit of an ass, Gear’s found that a surprising amount of her local patients have been asking for her help. This has made Gear happy since she gets to help ponies and use math.  
Her winter coat is thick and fluffy, during the spring when she sleds; the amount of lint rolls she goes through is crazy.