Is sacrificing a horse permissible or not? May Allah reward you.

Sacrificial animals (Uḍḥiyah) cannot be horses. Rather, they must be from the An‘ām (grazing livestock): camels, cattle, and sheep, as Allah Almighty says: "That they may mention the name of Allah over what He has provided for them of the animal of grazing livestock [An‘ām]." The Prophet, peace and blessings be upon him, and his Companions only offered sacrifices from these types.