Dog Body Language - Tail Wagging
You're usually right if you assume that tail wagging indicates a friendly dog, but it's not always the case. Dogs also wag their tails when they are scared, agitated or unsure. A frightened dog may wag her tail low and between her legs as she weighs up her next move: "Should I fight, flee or go belly up?" An aggressive, angry dog may wag her tail high while she chases or even attacks. Look at what's going on-is the dog's best buddy just getting off the school bus, or is another dog eating out of her dish? Also, check how the dog has distributed her weight, before being certain that the tail wagging is welcoming. If she's feeling aggressive, her body will be tense and the weight will be mainly on the front legs.