I used this photo and increased the leg length until aurochs-like proportions are reached, changed the body shape to a slender-waisted one as in other wild bovines, and created the shoulder hump the aurochs had. Subsequently, I replaced Londo's head with that of a Lidia bull, which I slightly blurred in length. Then, I painted aurochs-like horn tips plus a white muzzle ring that the Lidia's head originally lacked. I think the result is pretty accurate.
And now, see how much Londo actually differs from what we think a bull aurochs looked like (plus the size difference, which is not visible on this pic):
(Please mind that I used the Lidia bull's head without permission of the owner because I was unable to find that photo on google again. So please do not replicate it. If the owner of the photo is seeing this, I'd be grateful to know whether it is ok that I used it or not, many thanks.)
This fact inspired me to do some sketches using photos of aurochs-like cattle showing what an aurochs in the same stance and position would look like. I did not have the time for that yet, but it is about to come.