Aurochs

I'm reading the book ideas, a history of thought and invention, from fire to freud, by Peter Watson and at some point I encountered the word aurochs used in the plural. I thought it was a pity that the "proper" aurochsen hadn't been used, since words adding an -en in the plural are so rare in the modern language (oxen, children, brethren), but then I found out that both aurochs, unchanged, like fish, deer, moose, and so many other animal names, and aurochsen can be used for the plural, but aurochs seems more prevalent (not that either is that common, though).