I've always wondered how many animals can pass the self-recognition test, but either A) they don't do it immediately so they don't get counted as passing, or B) they just don't give a fuck.
Edit: or C) they get excited by their own reflection despite knowing it's themselves. Or D) they think you're doing some kinda crazy magic by showing them an image of themselves which is why they act like the mirror is showing a different animal (fear and/or aggression).