I've read some theories that our lion is a fusion of two or more brown lions. Maybe Rose's lions are a species of homeworld animals similar to lions, capable of fusion.
Cute episode but I felt the humor was weak, both old and new characters weren't very interesting, the history of Beach City is one of the lesser important parts of the SU mythos (can't say I can rewatch older episodes, see something, and shout, "Buddy's Book changes this!"), and I couldn't shake the feeling this episode existed only to hype unknown future eps. The part I really loved thou was when Steven and Connie see what Buddy really looked like. Reminds me of every time I read a book that gets adapted into a movie years later only to have the movie version of the characters look nothing like how I've imagined them.