Earth ponies, unicorns and Pegasus not are the only intelligent creatures on the planet. Further for example, zebras, gryfons, Diamand dogs, and more. Celestia kingdom, is only part of this world.
In the Equestria military are certainly represented in all three types of ponies. I am but afraid that their army is small, perhaps just as well as symbolic (Royal Guard). Their confidence in the protection of God's princesses is their biggest weakness.