A concept I had out of the blue. This is my head canon of what an earth pony guard would be. You don’t see them very often because they are both somewhat rare, with only a thousand or so serving, and they are also highly intimidating, so they aren’t assigned regular guard duties.
 
They are the largest breed of earth pony known in Equestria, with the tops of their ears coming almost to Celestia’s chin as reference. Additionally they are trained to the extremes of physical strength and endurance, resulting in a pony that can literally just walk through a stone wall if that’s what it takes to get at the enemy.
 
Temperament wise, because of their breed, they are typically very laid back when out of combat, not a whole lot will piss them off. Though if something does, there is no force on the planet aside from the princesses that will keep them from utterly destroying it. In combat they tend to go into a controlled rage if things get particularly brutal, and the more damage they take the more enraged they get, and the more it’s going to take to bring them down. Though they will always have the sense not to attack their allies.
 
As for the armor, it’s designed and built by unicorn smiths, magically perfected steel plate, which is then enchanted. It’s designed to be as intimidating as possible while still being practical and extremely durable. The usual gold plating is left out of the process in favor of treating the raw steel with various chemicals, giving a multi-tone appearance. One set of this armor weighs more than a unicorn guard and his full plate combined. The process for suiting up is incredibly lengthy, several parts are bolted together, and there are many layers.
 
The primary enchantment is cast by Celestia herself, hence her symbol on the breast plate. It’s entirely for convenience but requires incredible power to cast, the armor can transform into a medallion that the guard simply has to wear to suit up later. Unfortunately it isn’t as easy as enchanting this right away, so the initial suit up must be performed, and is done in an almost ritualistic fashion. The second enchantment is magic reflection, the armor will actively and very aggressively resist and repel magic attacks.
 
In short, they are trained to be, and their armor is purpose built to make them a living tank, they don’t even carry a weapon because they are the weapon.