Swan Queen

SwanQueen is the femslash ship between Emma Swan and Regina Mills in the Once Upon A Time fandom.

Canon
In Seasons 1 and 2, Emma and Regina were enemies fighting over custody of Henry. In later seasons, they became allies in order to fight various villains such as Zelena, Hades, and, in season 6, Regina's evil half, the Evil Queen. Said evil half came about via a Jekyll-and-Hyde split.

Fanon
Swan Queen is not canon as both characters have partners. This ship rivals the Outlaw Queen and Captain Swan pairings, which are both canon.

Trivia

 * Swan Queen won the Zimbio 2018 March Madness ship battle