My theory on royal guards (and why they all look alike) is that when they enlist they swear their obedience to one of the princesses. Those that swear to Celestia become the Day Guards, and the ones that do to Luna become the Night Guards. When they do, they get part of the Princess' power when they are "knighted" which changes their looks. Day Guards all become white coated with blue manes, and Night Guards get gray coats with yellow slit eyes and fuzzy ears. And if they are pegasus, their wings turn into bat wings.
The Dawn Guard, Princess Celestia's loyal warrior stallions. Along with their Princess, they watch over Equestria during the day. Favoring brute force and overwhelming numbers.
The Dusk Guard, Princess Luna's loyal warriors. Along with their Princess, watch over Equestria at night. Favoring stealth and cunning.
Both Dawn and Dusk Guards consider each other as comrades but there is something of a rivalry between them. Also to note, neither of these guard factions make any distinction within their own ranks over race (earth pony, pegusis, or unicorn). Although it IS more common to see unicorn guards in Canterlot.