Michelangelo's "Moses" has horns because of a mistranslation of a Hebrew word. The word actually means "shining," but a mistranslation in the Latin Vulgate version of the Bible led to a long misunderstanding of Moses's physical attributes.
The Vulgate was a fourth-century Latin translation of the Bible widely used for centuries in the Roman Catholic Church. Therefore, the idea that Moses had horns was widespread among early Christians, a notion which made some sense in context of other Mesopotamian religious figures who were often portrayed with horns. Because of this misunderstanding, many artistic depictions of Moses from before and during the Renaissance depict Moses with horns.