Feistiness is a characteristic of being spunky, assertive, or even a little aggressive. Your little pup's feistiness is clear from the way he snarls at the bigger dogs in the neighborhood.
Feistiness is from feisty and the American English noun feist, "small dog." A brave, spunky, and touchy little dog embodies this word perfectly — it carries both positive and negative connotations. Your feistiness can bring you success, because you're outspoken and brave. But it can also imply a quality of being quick to take offense and become aggressive: "My dog's feistiness got her kicked out of doggy daycare."