The Emerald Swans are a healing-type spirit beast. They are even on the verge of extinction, and are rarer than Darkgolden Terrorclaw Bears. The reason why they are about to go extinct is because they are very weak, especially at a young age. They are easily targeted as prey. In addition, their bodies are filled with natural and pure life energy. The life energy of a hundred-thousand-year Emerald Swan could even compare to Life Gold. They were considered great nourishment to any spirit beast.
As a result, an Emerald Swan was very likely to be killed before it matured into an adult.