Part of the origins of the European unicorn, just
like the ones of many other mystical creatures, lies in the Greek mythology.
The Greek god Zeus was nursed by the she-goat Amalthea (which was lateron
transformed into the star "Capella" (Latin: "goat") because of the gratitude
of the god). In one version, the god broke off one of her horns, which
spilled forth an unlimited amount of food. In later times this horn became
known as the "horn of plenty", which you can find on thousands of baroque
and classic paintings and ornaments. Even Uncle Scrooge is searching for
it in one of Disney's comic episodes, although it then "presents" not fruits
but gold coins.
However, as the unicorn is "known" to mankind for serveral milleniums another explanation of the origins of this creature is more than likely. As it is widely known the human race hadn't lived in nice comfortable houses for ever. Once it moved in clans and hunted wild animals for food. At this time the domestication of wild cattle was a incredibly important step for the first settlers for they didn't need to move with the wild herds anymore to gain meat. Due to the importance of these wild ox for the people at that time, traces of several "ox cults" can be found in the history of Babylon, Burma, Egypt or Israel for example. Just remember the story of the golden calf in the Bible.
However that is not the only quotation in this book. In the book of Hiob a creture called Re-em was mentioned, a name that was later translated into monoceros or unicornis, then to unicorn. Well, there it was doubted whether you can make the Re-em stay and work with all its strength and fury for you. Nowadays this might sound silly... but already for the people a few centuries later the cow was no more than a normal animal, a property. For them "Re-em" couldn't just be an ordinary cow, nor could that be that creature that could be seen gloryfied on the Ishtar Gate of Babylon. Therefore still facing these old oriental legends the Greeks and Romans started to believe that there had to be another mighty and fabulous creature (which made Re-em to monoceros in the Bible, too). For who would adore a cow?
And there was
one other thing. Have you ever seen Egyptian paintings? Remember how the
heads where shown without any "3rd. dimension"?. Well, just like that some
of the animals where painted "horn on horn" as you can see these antelopes
here. As well as in the ancient Corinth whoever looked on these paintings
that were on walls or vases for example could see but one horn. As this
way of painting vanished for the later peoples THAT became the unicorn.
Therefore, it is easy to explain why the early form of the European unicorn
looked almost like a goat and often was described having the same size.