Kyle would seem, from his behavior and basic body structure, to be a dog.  However, there are some very uncanine things going on with him.  Kyle is actually a little monster who seems to have some dog-like features, characteristics and behavior patterns.  He's not terribly friendly with strangers, and even can be a bit nippy and anti-social, from time to time, with his master Gru, with Dr. Nefario and the minions.  He even starts out that way with the girls (Margo, Edith and Agnes), but they quickly win him over after Agnes showers him with her irresistible charms.  He has since become very fond and protective of them.