There's a lot more to this incident that the anime glossed over. In the novel, the beast girls were huge bullies of the school, outright torturing other students and destroying all of their property. This was a equal and opposite reaction, as opposed to Rudy being a savage that is terrorizing others. Additionally, at this point in the book, no one reading knew that Fitz was Sylphy. But the people watching are fully aware, which is what makes Rudy come off foolishly.
Anyway, regarding the beast girls, because they are beast girls, survival of the fittest is a thing. Rudy showed them that he was the dominant by easily defeating them in battle (not the kidnapping), which is why they started referring to him as boss or master.