It can be done horizontal! The Howard Hill piece was horizontal. I can get boards with different lengths between the bark, but there is a limit to how wide. I have some that are 10 inches between the bark.
Also, I could request my wood supplier to try to find a board where the bark on each side is pretty parallel. Alot of the longer boards get narrower as you go up the board and may not look real good horizontal.
Do you have something in mind? Raccoon Skull, maybe? Or just thinking of the first trad buck sometime in the future?