I'm almost 100% certain about this. A Mary-Sue is a character that is way overpowered, and is perfect in EVERY SINGLE WAY
EX: Prettiest, Smartest, Most Popular, Everyone's Friend, Nicest, Never Mean, Never Angry,
If you were making a pony OC they could be alicorns that are perfect in every way
And most Mary-Sues dont have a past, and just have a personality which is perfect in every way.