I think Kovu wants to eat the ferret..

[Glamouresque]

Ferrets are good fighters, but I had a ferret and he lost one eye. He met with a old cat not used to ferrets and the ferret was biting the cat's legs and the cat reacted badly.
But I had other cats at the time and he was ok with them.

 

[Kristin]

I would try leashing Kovu around your ferret to curb bad behavior.
I have Zeddie around my parrots supervised. Sometimes she is in pounce mode but if I say no really loud and firm she makes a "mrrr" sound and runs away. The only time she really reacts to the birds is if they are doing something annoying (like screaming) or flying. If you have him on a leash, I would say no really firm when he goes to pounce, and pull back on the leash, or take him away from the ferret, then try again after a while.