Moyses

This form obviously originated from the Biblical Greek form Μωϋσῆς (Mouses). After all, it can be variantly transcribed as Moyses, because the Greek letter upsilon (also called ypsilon) can be transcribed as both 'u' and 'y'. That also plays a role into why the modern Greek form of the name is Μωυσής (Moysis):

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• https://en.wiktionary.org/wiki/%CE%9C%CF%89%CF%85%CF%83%CE%AE%CF%82 (in English)

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Not to mention that the letter 'y' is not native to the Latin alphabet and was adopted by the Romans after the Roman conquest of Greece in the 1st century BC, likely in order to more accurately transcribe words and names of Greek origin (or that were at least from sources written in the Greek language):

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• https://en.wikipedia.org/wiki/Latin_alphabet#Classical_Latin_alphabet (in English)

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All in all: Moyses is not a genuine, native Latin form of the Hebrew name מֹשֶׁה (Mosheh). It is more like a (variant) latinization of its Greek form Μωϋσῆς (Mouses).