An image that alludes, spoofs, or otherwise parodies the oftentimes complex real-life familial relationships of the racehorses that the Umamusume girls are based on.
Note that the only familial relationship that is acknowledged canonically in Umamusume is the one between siblings (born from the same dam). A good example for this is the V Sisters. In their case, it would only be an example if any of Halwa Sweet, Deep Impact or Heart's Cry are also present.
The name of the Pixiv tag The Dark Side of the Horse refers to the complicated bloodlines of racehorses as a result of their breeding procedure, in which a single sire has several partners, with incestuous pairings being common to further strengthen the strong bloodlines.
