What Breed Is My Dog?

She is very friendly with everyone, but not with any animals. She becomes too agressive when she sees any dog, monkey, cat, etc. She is very active and fast. She is also a very good guard for our home. Her height is 19 inches from back bone, 25inches from ears, and 27 inches wide from muzzle to tail. And last thing she is a mixed breed. Now please someone tell me which breed she is?