I also got this from my friend who loves her irl... This is just theory, but the reason why she folded her right leg here is not only for the sake of a cute Marilyn Monroe act, but also because irl Win Marilyn actually has a recurring right elbow hygroma (肘腫)—a fluid-filled swelling on the point of the elbow caused by how she folded her legs when lying down in her stall. She got it because Win Marilyn has a stubborn habit of folding her right foreleg tightly under her chest when she lies down, causing her hoof and horseshoe to press directly against the point of her right elbow. So if we follow that kind of habit into the image with right foreleg as her right arm and her leg and you get this Monroe pose that she show in image.