This is ID's original clay figure that they captured for animation [link] . small details are usually lost in limited colors and lower resolution. The Barons of Hell also have ears on the Doom cover, but not in the game. So I'm not sure if small details like that matter too much when you draw a more realistic scene.
It's ok. Drawing fantasy/game characters more realistically means having to add more details or change things slightly in order to match your style or a theme.