Yours, Forever After by Beth Bolden

Fifteen years ago, Prince Graham of Ardglass barely escaped from the ancestral castle with his young life. Rescued by a magical creature and spirited off to a faraway valley, he grew into a strong, capable man—never shirking his duties on the farm, but forever bitter over his father’s betrayal. But just when he has finally come to terms with being lost and staying lost, a visitor arrives in his valley and changes everything.
After a lifetime spent lost in his beloved books, Prince Emory awakens to find his villainous aunt working to usurp the throne of Fontaine. When she sends him on a dangerous quest, he’s certain the journey is a trap, but he’s not willing to accept defeat without a fight.
But a fight is something Rory is unprepared and untrained for, until he’s saved by a handsome, unassuming farmhand and his snooty, smug, and surprisingly talkative unicorn.