I have raised dogs all my life and have had some pretty ferocious ones. The meanest have to be the Chihuahua's those little shits will bite ankles with good behavioral training. Right now I have a Mini Dachshund that is really good but she came from a no-kill shelter and must have been abused or hurt as a puppy because she does not like dark clothed people she will go batshit crazy barking if she doesnt know you or we dont tell her we let them in.
