The Hyach-do are an extinct species which were closely related and genetically-compatible with the Hyach.
The Hyach and Hyach-do diverged from a common ancestor about a million years ago. At first, the two species competed for territory and resources, but at some point learned to coexist. They interbred frequently, supplying genes that made the Hyach more fertile. However, such unions were regarded as scandalous. In the 11th century, religious laws made them immoral, then illegal, and finally punishable by death. Not satisfied with this, the Hyach launched a genocidal campaign against the Hyach-do which took centuries, and sentenced to death anyone who tried to help smuggle Hyach-do off-planet. About 400 years later, the Hyach-do had been rendered extinct.