Are the Pharaohs, who worshipped Horus and the sun and to whom Islam did not come, considered disbelievers?

Allah does not punish anyone before the حجت (proof/argument) reaches them, as He says: "And We would not punish until We had sent a messenger" and "so that mankind should have no argument against Allah after the messengers."

If no messenger was sent to the Pharaohs, or if the remnants of the messengers' call did not reach them, then their ruling is the same as that of the people of the Fatrah (interval period), who will be tested on the Day of Resurrection.