Yamata no Orochi was a project, and the Master of Ten Thousand Transformations was the final result of that project.
For example, a little boy is an atomic bomb, but not all atomic bombs are called Little Boy.
"In summary, after the newly-born Master of Ten Thousand Transformations slipped out of control, the Emperor of Heaven Government invested a massive amount of manpower and resources to continue pushing the project. But in the end, not only did they fail to succeed, but they also attracted the attention of certain Ancient Deep Sea Gods. Their will caused severe contamination to the newborn Orochi, leading to its complete loss of control."