Every animal with a mind has the capacity to lose hold of it from time to time. Sometimes the trigger is abuse or mistreatment, but not always. I've come across depressed and anxious gorillas, compulsive horses, rats, donkeys, and seals, obsessive parrots, self-harming dolphins, and dogs with dementia, many of whom share their exhibits, homes, or habitats with other creatures who don't suffer from the same problems. I've also gotten to know curious whales, confident bonobos, thrilled elephants, contented tigers, and grateful orangutans. There is plenty of abnormal behavior in the animal world, captive, domestic, and wild, and plenty of evidence of recovery; you simply need to know where and how to find it. Oliver was my guide, even if he was too busy compulsively licking his paws to notice.
Acknowledging parallels between human and other animal mental health is a bit like recognizing capacities for language, tool use, and culture in other creatures. That is, it's a blow to the idea that humans are the only animals to feel or express emotion in complex and surprising ways. It is also anthropomorphic, the projection of human emotions, characteristics, and desires onto nonhuman beings or things. We can choose, though, to anthropomorphize well and, by doing so, make more accurate interpretations of animals' behavior and emotional lives. Instead of self-centered projection, anthropomorphism can be a recognition of bits and pieces of our human selves in other animals and vice versa.