Huli De Xianjing

Since the age of ten, high school girl Yan has been plagued by a dream in which a fox brutally devours her heart,
Her nightmare almost comes true when Ran, a beautiful demon fox, approaches her during a school field trip when she's left alone. Much to her surprise through, instead of harming Yan, Ran insists on returning her the favor for finding his lost tail. And in exchange for this tail, the omnipotent demon fox grants a wish to the girl.

(Source: Pocket Comics)