It's fairly common for multiple systems to have a few animals or non-humans in the system. Some systems are entirely comprised of non-human entities.
Residents in a multiple system may self-identify both as animals (in any or every way, whether talking animals or with animal communication abilities, etc.), or as anthropomorphized versions of animals or hybrids, aliens, mythological creatures, objects, androids or robots, artificial intelligence, etc.
Even from a trauma-based multiplicity point-of-view, it makes complete sense to develop or call upon non-human headmates when humans continually fail you or make you unsafe or hurt you.