Looking into animal behaviour

I'm looking into animal behaviour as I want to see what I am able to incorporate into my character. The way an animal goes from their normal state to defensive is quite interesting as their whole body changes, not only the face. Some animals will puff up their fur or feathers to make them appear larger than they seem, whereas others will snarl and roar.

On the right you can see cockatiels, they will puff up their feathers and on the top of their head they have other feathers which will stand up on end when they become interested in something. Four legged mammals such as tigers, lions, dogs, wolves etc will tend to lean into the direction of the threat. Looking at the pose of the tiger it is far more dynamic and powerful - there is more movement within the whole body. I think a four legged animal or creature is the route I might take as it allows more scope for dynamic poses and movement. This tiger is in a more defensive pose, however with my character it will be on the offense so more than likely it will lean into the direction of the photographer. The movement will have to be quick and powerful and I will also have to make sure that the construction of my character will allow me to push it's body further.

Pixar's Brave came out in summer and looking back on the film it seems quite relevant to what I'm trying to achieve. In the film there are two bears, one gentle and kind the other aggressive and powerful. What's interesting about the bears is how Pixar distinguished between the two, by pushing back the ears and contorting the face and making the pupils smaller the bear drastically changes from a sweet and harmless animals to something far more ferocious.
 
“By changing the closing of the eyes, changing the scale of the iris and pupil, it goes from Elinor as a bear to…a bear.” - Bill Sheffler, character supervisor on Brave

Now the problem I need to solve is that real animals going from their normal state to defensive don't tend to change too much, I want a character that looks really cute and innocent to begin with yet will change drastically. I need to find a good balance the cute small bears shown on the right and the angry snarl. There needs to be a powerful transition yet also needs to feel believable that my character could get so angry.