This really makes me want to get into Terramagne and write about a character with depression and horse traits. Because ketamine's main use is, in fact, for large animals. It's safer for them than a lot of other drugs -- particularly for skittish prey animals who are otherwise prone to spook and bash themselves into walls or trample nearby humans. So it stands to reason that while antidepressants might work much less well on equine-human persons, ketamine might work quite a lot better. That doesn't necessarily mean it has no side effects, but it's probably a reliable way of damping down the depression. One of many ways in which soups, especially primal soups, can have very different reactions to stuff than ordinary humans do. And yet, truth told, it works well enough for humans that some stick with it like glue -- especially people for whom the usual stuff does not work. So then, an equine primal, and their human friends who also use ketamine, and their support group, because T-America doesn't bury this stuff under the rug near as much as here.
Or we could always go with Moon Door instead. You know some of the Women's Chronic Pain Support Group must have tried it, some of them are try-anything types. My money's on Soma.