There is a reason
oxen look like cows, or more specifically, steers. Any steer yoked to pull something is an ox. Oxen are not a breed or bovine species. Oxen came from the same place the cows came from.
Another quote from Caesar, same site...
The horns, in amplitude, shape and species, differ much from the horns of our oxen.So. Aurochs and tamed bulls co-existent. I wonder at what point one was tamed, and the evolution of oxen different in appearance from the Aurochs?