Ive just gone through this with my whippet and 3 terriers. I got them to the cage first(one at a time) and took the ferret out saying mine which means mine, I use this to take everything off the dogs. Then slowly started letting the ferret on the floor and stood by the dog again saying mine, they soon get it. Theres only my border that I can't trust, I thinks its because hes had rats. ATB John