Actually as I think about - the best way to go with the Primal "Animal" aspect - is this. Many native cultures believe that each person has their own individual "spirit guide" - which is basically an animal spirit that helps to guide them on their life path. In these native cultures -people would often have to undertake some kind of ritual spirit quest - to find their animal spirit guide.

For a Slayer - Their specific animal spirit is also often revealed during a ritual spirit quest. However with Slayers -since the First Slayer assumes the form of the specific type of animal in question - that is referred to as the Slayer's Aspect.

So for Buffy - her aspect would be a Cougar or Mountain Lion - since that's the animal that First Slayer used to guide Buffy during "Intervention". And actually a Mountain Lion is a fitting aspect for Buffy - since Cougars/Mountain Lion are usually solitary hunters.

For Kendra & Faith - their aspects would have to be guessed at. For Kendra - my guess would be a Wolf - Since Kendra virtually always seems to follow the orders of those she considers to be her superiors (pack leaders).

Faith is also a loner, like Buffy, only more so. A good guess for her aspect - could be either a Bear or a wolverine.