Concept art of Ustiarius, royal guard of Canterlot Palace in my fan-fiction, The Tale of Two Sisters. Fun fact: Ustiarius is Latin for "Doorkeeper".
Ustiarius is designed to be a generic royal guard, thus his fairly simple design, accented with some facial hair for personality. The armour designs are just ideas, and I've yet to nail exactly what I want the guard armour to look like; it needs to be reminiscent of that seen in the show, with an archaic twist, somewhere between medieval European and Roman. For context, my head-canon for the eras of design go: Pre-Classical Era, aka medieval (current story era) → Discord Era, aka Roman (think Flash Magnus) → Classical Era, aka Grecian (I think the comic covered this era) → Modern (show-style). I'm aware that this isn't chronologically accurate for humans, but it fits with the show's canon. You'll also notice I've not settled on Ustiarius' cutie mark yet either… but I wonder if you can see a clue in the designs.

Originally uploaded: August 30th, 2018

While these colours were accurate until now in the story, I have more recently decided to change Ustiarius' colour palette, as I had too many grey/white ponies interacting (I don't design with colour in mind). Despite intending his palette to be a generic guard's (white coat, blue mane), his new palette looks set to be brown/brown, which should contrast better beside Starswirl and Celestia.