True Dragons are the real Dragons with pure ancient bloodline of the Divine Beast. In their eyes only pureblooded has the right to call themselves Dragons. However, many various beasts like Flood-Dragons and Dao Serpents, who had a bit of True Dragon's blood boasts to be Dragons as well. To True Dragons, however, all these beasts with diluted bloodline are mere insects. To distinguish from them the real Dragons had chose to call themselves True Dragons.
True Dragons possess the strongest and most precious bloodline in the world. However this strenght is also a curse; their bloodline is so strong that it's very difficult for them to reproduce. Moreover, their blood is a high-level medicine for life-prolongment, so strong people wants to hunt them.