Humans make the most ubiquitous monsters ever. Good old human beings. They exist in just about every setting (sci fi, fantasy, etc), probably because all of these fictional worlds are created by human beings. If someone knows of a fictional world not imagined by a human being, please pass it on to me.
I do think that some notable exceptions to worlds without humans are those that have human equivalents - like anthropomorphic animals, or human-analogy robots, etc.
Humans are everywhere, they have every sort of motivation, they have access to all of the weapons and technology and magic that the players can access (usually), and they often have access to the same game abilities as player characters. [I would guess that some exceptions would be in something like Vampire, where humans are definitely below the level of the monsters. But no sensible gamer would associate with Vampire players anyway. Sheesh.]
Now, keep in mind, that although they are common and quite flexible, I would turn down humans every day of the week for orcs. Much cooler (see below posts on the king of monsters - the orc).