What is Happy Tail? This is an injury to the tail usually caused by wagging the tail up against something sharp or rough. The tail may bleed, split open and sometimes the bone in the tail actually breaks. It is very common in Greyhounds and other breeds without much hair to pad their tails. Tail injuries bleed a lot and each wag can throw droplets of blood from floor to ceiling, creating a bloody scene worthy of CSI. You can't imagine it until you actually experience it.