Just to add to the fun, the etymology of the word "dog" itself is completely unknown.
There's a single documented mention of the Old English "docga" - and then nothing.
How this word took over pretty much well entirely from "hound" is a mystery.
If that's not enough, the Polish word for dog - "pies" - has exactly the same issue!
(and I believe, but can't say with any certainty, that other Slavic languages may have the same issue.)
Edit: Oxford English's page on "dog" is paywalled off but still available in via Google's cache, a fascinating read if you have a spare 15 minutes:
http://webcache.googleusercontent.com/search?q=cache:jNMSJSG...
