User blog comment:IamJakuhoRaikoben/Chapter 331: Natsu's Strategy, Review./@comment-5708675-20130517210057/@comment-7418318-20130519162939

Most likely they are a category of their own. The question is, wether they are similar enough to any of the other three categories that they might occupy some grey area enabling one of the siblings to take them over.

I think animal is the least likely. While they might have originated from animals, with their supernatural innate abilities and affinity for magic, they must be closer to beasts and demons by now. Also there was the engraving on Lisanna's grave "Friend" of animal souls, which leads me to believe that take-over animalsoul is somehow cooperative or symbiotic, unlike the other two "defeat and steal" take-overs.

And not all beasts are automatically stupid. The Vulcan for example (forrest varient at least) seemed to be on par with the lower ends of the Human genepool (he clearly outsmarts Zato, Gato and Toby Horhorta.... and the FT members at the beginning of the KotSS arc -_-)