This creature was named Rivar. From his surname, it is clear that he was the leader of The Syndicate Organization, as well as the strongest if The Syndicate Organization and Twelve Islands are included.
The power of Rivar is peak Mage King. It can be said that he was stronger than Nelion if it came to pure magical power because of the level difference.
But of course, in a battle-scenario, everything may be changed. The thought process of Rivar is different and understandable.
After Rivar had taken the lead of The Syndicate Organization, which was not The Syndicate Organization back then, he had created a family line. As a surname, they were using Monsterim. It may sound weird, but it was not that weird.
In the end, Rivar just gave the name of the organization, and why would he not name the organization name after himself?