A friend of mine once named his dog "cat" and his cat "dog". It was hilarious to watch the look on peoples faces when he called for his cat and the dog came running.
i have a pretty good size rottweiler. i named her tinkerbell just for the irony(spelling?). its pretty funny to have somone come over and say "ive got a dog nammed tinkerbell" then see the look on there face when a rot comes around the corner.