An idea I've been playing around with based on armor of 13th century European knights, which pretty much just consisted of mail armor and a surcoat (or caparison in RD's case). Despite being so simple, it took a while to settle on a color and look that matched the style of the show, especially the mail texture and the details on the caparison.
It's worth noting that the halberd she's holding wasn't in use until the 14th century, as it and weapons of its type like the pollaxe were developed to pierce the plate armor which was being worn with increasing commonality. I just thought it suited her well
Inspired by: [link]
Spear courtesy of [link]
And I've got Rainbow Dash wearing it because...well, why not?