Weee, kinda feels like this is going "kitty became the king of panther monsters,look at how all the other monster panthers follow it into battle against the goblin king and his goblins!"
(Would also lessen the outnumbered issue, besides feeling like the most natural direction of taking the story at this point.. Would be more surprising if it didn't.)
And answer to the title question is probably a desperate desire to protect or something, like protecting the goblin society for goblins to get their king. (adventurers overhunting?)
If I'm right we'll see if I'll get pestered with accusations just like back on batoto. =p
Also, picking panthers for four-legged monsters here feels rather odd unless it's to become cute/fluffy strong allies or something somehow.
Usually writers pick something like wolves/wargs or something else that feels more native to the region and aint as cute/fluffy without extensive modifications.