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 single sire has several partners, with incestous pairings being common to further strengthen the strong bloodlines.
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.
So far, there are many father-child pairs (i.e. Rudolf and Teio) but no mother-child pairs in Umamusume. The few mother-child pairs that have any chance to appear are Cesario and Epiphaneia, Fusaichi Pandora and Almond Eye (as the main story seems to go in that direction) as well as Air Groove, Admire Groove and Duramente and Air Groove and Rulership.
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.)
