User blog comment:Lux capacitor/Possibilities In RWBY - Animals/@comment-24087318-20141014035645

Animals exist. We know this. See: Zwei, and even before that. As for wether or not they can use/gain Dust powers. It's fully possible: Dust reacts with, and is triggered by, a persons Aura. We know Animals, plants, and pretty much anything Non-Grimm has an, at the very least, inactive Aura. Given the possibility of an animal having their Aura unlocked, they could, with training, be taught to utilize Dust.

And that's pretty much all there is to say on the topic. Wether or not non-mundane animals/creatures exist in RWBY, like Pheonixes, Unicorns, etc, that arn't Grimm based remains to be seen, and can't really be hypothesized accurately.