Dragons actually went "Moo." In ancient times, cows went "RAWRGRAAAARGH!" Most members of the dragon race were attracted to cows more than their own kind, so they started mating with them. This caused dragons to later become extinct. Early Cowdragons were fierce, breathing fire and making loud MOOOOO noises. The dragon blood thinned for every generation of Cowdragons, yet some qualities from dragons remained. This left us with the modern cow, docile and making soft and cute "moo" noises.'
The misconception that dragons roar is based on early accounts of early humans encountering dragons mating with cows, with the cows making roaring noises. Confused and frightened, they thought that the dragons were the ones roaring and that they were hunting the cows. They ran away, scared. Only after many centuries would humans find cows again, with extremely thin dragon blood, mooing and generally being retarded.