This is what I imagine to be the earth pony counterpart to the Alicorn Amulet: a living suit of armor that is fully alive and sentient and able to move of it's own accord.

Aside from the protection she offers her wearer, she also syncs her movements with theirs to bolster their strength to an intense degree. The downside, of course, is the strain this puts on their body and her ability to overpower her wearer and move his/her's body. Needless to say, in the heat of a battle, every one of these attributes is a blessing and a curse :D