From the description:

“Applebloom was the Ponyarch of the Flank Eaters, a legion filled with an extremely savage bunch of fillies. The Flank Eaters had a two rules that everyone had to abide in order to qualify to become a Flank Eater.

1st RULE: You must not have a CUTIE MARK.

2nd RULE: You MUST NOT have a CUTIE MARK.

It was however inevitable during their campaigns that fillies got their cutie marks through certain spectacular and most often very brutal events. In order to solve this appearent problem, they all wore armour that stretched over their flanks, hiding any cutie mark that could be evident there.

Applebloom herself was extremely impatient and always lusted for combat. Nopony knows If her frustration is due to not having a cutie mark, but she sure is angry! And yet to this day, it still remains unknown if she has acquired her cutie mark behind that leg armour, or not.”

…And I for one do not have the balls to find out.