In one of Harley Quinn's introduction lines to Mr. Freeze, she asks if he can "make her a pony", to which he retorts, "It will make the twilight sparkle." Harley Quinn then will say, "Hahahaha! I was hoping you'd say that." This is a reference to Harley Quinn's voice actress, Tara Strong, who also voices Twilight Sparkle in the series "My Little Pony: Friendship is Magic".
Contributed by RyuHane