I suck at art, so I'll try and make it up through the bio. If I don't win her, I'll probably just transfer the bio to another pony OC.
Name: Minim, after the musical note. Of course her cutie mark isn't actually a minim, but it just sounded better than 'Hemidemisemiquaver'. From an in-universe perspective she was supposed to be very fast. Like, moving from place to place in half a beat kind of fast.
Personality: Minim is a very tough kind of pony. She hates sappy stuff, though she generally channels her emotions through music. Generous, spunky, and sarcastic, she never backs down from a challenge.
Likes/dislikes: She loves music, unsurprisingly, but she also has a great fighting style due to her love for it. While you can expect many other rock musicians to be good at combat as part of their image, she has a much more trained style. They might just hit and punch whenever they can; she is opportunistic and knows where to strike.
Minim, oddly, hates unnecessary violence. She can actually stand classical music, but she despises anypony who thinks they're all superior because they play or like it. She also dislikes standing still, due to her boundless energy.
When Minim was born, it was planned that she would be drafted into guardship, and it was instantly decided that she would be raised to learn how to do the job. Her father taught her basic fighting at first, a task which she enjoyed. It stretched her muscles and gave her much physical ability.
Really, she would have stuck to the job if not for the extreme discipline needed - her energy, despite trying to find ways to quell it, was limitless. When she was supposed to stand still, she would always find a way to distract herself - usually tapping a hoof against the ground in a steady beat.
And it came that eventually her mother brought her to a rock concert in Manehattan in order to, as she said, "Get this damned hyperactivity out of your system." It was expected that once it was over, the metaphorical fire that burned to move would be satisfied, and she would finally learn how to stay still.
However, this was not to be. Instead, she gained her cutie mark - a hemidemisemiquaver. (Not that she knew what it was at the time...) She knew enough to figure out that it was a music note, and she showed it to her mother in excitement.
This happened to have a different reaction than expected; Minim's mother was horrified at her child having such a destiny of undisciplined sloppiness. While she was far too honorable to go so far as kicking her out of the home, Minim was treated with a certain degree of coldness, and the royal guard training ended.
Again, though, her parents were the type of ponies who refused to be cruel - they were royal guards, after all. For knowledge that most ponies had jobs related to their cutie marks, and that she would need training to get such an occupation, she was sent to music classes.
Of course, seeing as it was Canterlot, there were few rock lessons, and she could only learn classical music. This, of course, was not her forte, and Minim typically spent half of it making up happy beats uncharacteristic of the genre on the timpani. Her skill in this was surprisingly high.The instant she could, she moved out of Canterlot and into Ponyville, bringing with her a large amount of bits... She couldn't survive alone, after all. After getting herself a small cottage, Minim bought herself a drum kit, knowing this was her destiny.