http://www.uesp.net/wiki/Morrowind:Boss_Crito is a boss for lower levels. He even has the name for it
Despite the name, I'd say that character isn't unique enough to qualify as a boss. I mean, he seems to pretty much be a generic bandit, no unique model, absolutely no personality or background, no dialog, no unique items, nothing. In general, I'd only qualify a character as a "boss" if that character has something setting him apart from generic enemies.
Although, even the enemies that I'd actually consider to be "bosses" in Morrowind don't really feel like it, I mean characters like Dagoth Ur, the Ash Vampires, and maybe some of the notable characters you fight during the game. I call them "bosses" because they're unique characters you have to defeat to advance the story, and are usually somewhat stronger than generic enemies, though you might hardly notice this, depending on what level you're at when you fight them, but they don't really FEEL like boss battles. Usually I expect a boss fight to be an epic fight against some powerful enemy. Usually, the fight should feel different from generic battles, not necessarily because you need a unqiue strategy, but the boss should usually use abilities or tactics that other enemies dont use. The effect is generally best if the fight is in unique surroundings and accompanied by special boss music, and Morrowind just doesn't capture that very well. The differences between bosses and normal enemies are usually just cosmetic, the only actual gameplay differences are generally that bosses take longer to kill and can kill you quicker.