When an image alludes, spoofs, or otherwise parodies the real-life familial relationships of the racehorses that the Umamusume girls are based on.
The Pixiv tag's name refers to the complicated bloodlines of racehorses as a result of their breeding procedure, in which a horse (typically a stud i.e male horse) has several partners, with incestous pairings being common to further strengthen the strong bloodlines.
Pixiv equivalent tag: ホースの暗黒面 or 史実親子 (parent and child in real life)
(Yes, it's also a Star Wars reference.)
