But since this bizarre idea has enjoyed quite a bit of dissemination in the past, it is worthwhile to ask: just how did this whole misconception get started?

Let’s go back to the Bible.  Moses was on the mountain top to receive the tablets twice.  (Remember, he broke the first set and had to return to receive a second copy.) 

When Moses descended from Sinai the second time, the Torah tells us¹   “the skin of his face was radiant.”  In fact, to protect the people from being overwhelmed by his glowing aura, Moses put on a veil at all times, except when teaching Torah.²

It is unknown whether or not Michelangelo understood the Biblical verse properly and was just trying to represent beams of light in stone or if he actually thought that Moses had horns

So rather than meaning, “to emit rays,” he understood it to mean “to grow horns.”  (You can see the connection between the two ideas and why the etymology would be similar, but context makes it pretty obvious that we’re not talking about antlers here.) 

When Michelangelo sculpted what he imagined to be the likeness of Moses, he included odd protrusions jutting out from the sculpture’s forehead.  It is unknown whether or not Michelangelo understood the Biblical verse properly and was just trying to represent beams of light in stone or if he, like Jerome, actually thought that Moses had horns. 

What matters is the popular perception and that to most people, these strange bumps sure looked like they were supposed to be horns.  From there, this likeness became widely accepted and was copied by other artists spreading the misunderstanding far and wide.