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 01 856, which I slightly edited. Then, I painted aurochs-like horns. 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):
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.