[F Mushoku] Fallen Horned Owl, Cuckold Bride (Bessatsu Comic Unreal Ishu NTR ~Ningen ni Koishita Jingai Heroine ga Douzoku Chinpo de Kairaku Ochi~ Vol. 1) [desudesu]

Mel, a horned-owl harpy who has served her master's family since the day Shou was born, cannot understand why the boy she raised has grown so distant — and a dark-skinned beastman of her own kind knows exactly how to exploit that doubt. Dragged into a forceful mating that overwhelms the supposedly extinct species' strongest female, the once-loyal birdfolk bride abandons her human first love for a same-species mate. The epilogue finds Mel heavily pregnant and blissfully begging her new hubby to breed her again, while Shou can only mourn the companion who left him.