Finding the psychotic yorkie story hilarious, funnily enough the most dog aggressive puppy in our class was a chihuahua called Romeo of all things, it was a right little sod. Trainers insisted it was muzzled at all times in training after it bit two of the trainers and attacked 3 dogs in a single hour training class. It would be sitting quietly minding its own business, and bam, next thing you know it was hanging off someone's hand, or off another dogs neck. This was in a class of around 15 dogs, about half of them staffys. The woman insisted that it didn't need a muzzle as it was so small it wouldn't hurt anything, trainers basically said muzzle it or don't come back because if it goes for one of the bigger dogs and it turns you will have a dead chihuahua puppy and it will be your fault. It bit her the following week and she had a muzzle on it the week after