Q: Moses had horns like a devil. Exodus 34:29, says so. why is that, do you think?
A: Moses did not have any horns, that is, unless you are reading the Catholic Bible -- Douay-Rheims Bible -- basically, the Latin Vulgate translation handed down from Saint Jerome, around the 3rd-4th century. Here is the quote that you are most likely making reference:
(Exo 34:29) And when Moses came down from the Mount Sinai, he held the two tables of the testimony, and he knew not that his face was horned from the conversation of the Lord.
This word, horned, should have been translated, radiant, but simple mistake led to the famous statue, by Michelangelo, depicting Moses with two horns coming out of his head. Yes, even Michelangelo can be wrong. Check any other translation and you will see that this error has been corrected.
(Exo 34:29) And it came to pass, when Moshe came down from Mt. Sinai with the two Luchot HaEdut in the yad Moshe, when he came down from HaHar, that Moshe had no da'as that the ohr (skin) of his face had become radiant when he had spoken with Him. OJB -- (This Orthodox Jewish Bible, says it best, NIV, ESV and KJV all agree)
(Exo 34:29) When Moses came down from Mount Sinai with the two tablets of the Testimony in his hands, he was not aware that his face was radiant because he had spoken with the LORD . NIV (LORD should read, YHWH, or YaHWeH – His Name replaced by the Translators)
The Hebrew does say to "shoot out horns", but this is to say, "shoot our rays", as bright light shoots out in many different directions. The context also makes this clear. It was difficult for the people to look on him because of the brightness of his face.