Medieval Christians usually called Muhammad a prophet or pseudoprophet, not a god.

Evidence

Muhammad’s claim to be no more than the messenger of God is implicit in the words used by a wide variety of authors. Propheta (with pseudopropheta) was the word most often used, but words which more nearly translated rasul than nabi were also used — messenger or envoy (nuntius) and apostle (apostolus). Combined phrases are found, propheta vel nuntius or propheta Dei et nuntius. — Norman Daniel, Islam and the West, 116