True, mine was also fixed when we got him, I'm not even sure if Sid was a ferret because he was the most low energy pet we've ever had lmao
It's still possible to buy ferrets that aren't fixed specially from third parties so I just thought I'd ask, maybe it's just Ernie's personality to test the other animals and see how far he can get before being put in his place and understanding that not everything is prey. Maybe give your puppy another 2-3 months before letting them interact without the cage, I wish I could give better advice but ferrets are so stubborn that I can't think of any training exercise that might help