Most of the time a wagging tail is a good thing, but not always. Be mindful of these signs that a wagging tail may mean trouble:
1. A slow, low wag: this type of wag can be a sign of insecurity. If a dog is feeling especially fearful, the tail may continue to wag even if it’s tucked between the legs. If you think a dog is feeling uneasy for any reason, be very cautious in introducing a new person or animal.
2. A slow, high wag: on the opposite end of the spectrum, a slow, high wag is often a display of dominance. Caution should be used if a dog is exhibiting this behavior.
3. A wagging tail accompanied by barking: this is often a warning sign of aggression or, at the very least, over-excitement.
Keep in mind that dogs hold their tails at different heights depending on the breed. A broad wag side to side typically indicates a happy pooch, especially if the hips are wiggling with the tail. The best way to tell if a dog’s tail wag is negative or positive is to observe his or her overall body language. Happy dogs generally seem relaxed, are holding their tails at their natural height, and aren’t showing any signs of anxiety or aggression. Of course, the best way to determine if it is a positive or negative wag is to know your dog. Observe your dog’s behavior in different situations so you can better predict their state of mind.
*This was taken from an article found HERE.