The national park was submerged due to rising water level of river Brahmaputra, triggering floods.
As per reports, the horses were swept away at Kobu Chapori in Jonai from the national park.
A forest official said that two forest villages of Dibru-Saikhowa National Park, Laika and Dodhia, have been flooded and many people are affected. Many of them were also shifted to relief camps and other safer locations.
He further said that the national park gets flooded every year, adding that no rescue operation has been started yet.
Dibru-Saikhowa National Park is located in Dibrugarh and Tinsukia districts of upper Assam.