Dang. This manga hits me with the wholesome feels out of now where. The whiplash is strong with the general horror vibe most of the time.
I've been thinking that the horse thing is maybe a representation of any negative feelings the family has in the household, as any household would have some amount of for any number of personal and collective reasons.
But while a bit unsightly aren't being dangerously repressed or angrily vented at each other so the its just a mostly benign presence. Could become an issue if left to fester, but the family dynamic seems healthy enough for the time being.