My girl Bean started out VERY defensive when I got her. Soon as the tub opened she was puffing, hissing and attempting to strike. The key is going slow and being consistent. It's perfectly fine to let them tag you, defensive bites really don't hurt at all, and they'll start to learn biting doesn't make you go away. Move slowly and calmly, pick the animal up with confidence, handle it long enough that it starts to give up and accept it's fate, then put it back. You're teaching it that being calm and peaceful is what gets the result they want, ie. giant squishy tree puts me down and goes away.