I do this a lot, wonder about word origins, or phrase origins, and then look them up. Sometimes in the middle of the night when I can’t sleep 
:
‘A unicorn isn't called a "unihorn" because the word comes from Latin (unicornis) meaning "one horn," borrowing "uni-" (one) and "-cornu" (horn), and English adopted the whole word wholesale, even though "horn" is Germanic, creating a Latin prefix with a Germanic root, making "unicorn" the standard, descriptive, and ancient name. "Unihorn" is just a modern, less common variation, as the original myth adopted the Latin-derived term long ago. ‘