I read this novel in paperback around 2002 in the UK, I bought it new then so it was in print in the early 2000s.
The novel was set in Roman times, but I cannot recall during which period or Emperor. I think it involved Roman Britain so at least Hadrian and possibly Antoninus Pius.
The main character was male and, I believe, a werewolf. He was not Roman, but wasn't actively against them. As I recall the character, he was focused on his own life/mission and only came into conflict with the Romans when they got in his way.
The cover was predominantly black with a man's face and a wolf's face (head?).
It is very hard to search for anything using the above in Google that doesn't result in you receiving results solely about romance novels!