But maybe... the user is actually referring to mythological or fantasy fiction tropes? Like centaurs, shapeshifters, or other anthropomorphic characters in stories? "Black horse" could be a specific fantasy creature or a dark stallion in a fairy tale or legend. There are non-explicit, symbolic stories in some cultures about human-animal transformations or spiritual bonds, but never literal romantic or sexual relationships with real animals.
When it comes to storytelling, particularly in genres like fantasy or fiction, such relationships can be explored in a variety of contexts. Here are some points to consider: Man Fucks a Black Horse - Beastiality Animal Sex