hearing about the possibilities of werewolves, vampires, and suspicion makes me wonder about other sorts of enemies that might be ruled by suspicion, enemies that in some way act like normal dwarves until you or your dwarves start to notice some alarming patterns...
Dissenter/Rabble-Rouser: A dwarf that provokes otherwise happy dwarves to tantrum, strike (take breaks en-masse) or bother your nobles. A rabble-rouser might have demands viewable on z-thoughts and stop rabble-rousing if these are met (such as private room, work, etc), or nobles who grow suspicious enough might track down the actual problem dwarf and have him punished. Rabble-rousers might arrive as migrants, or might be fortress natives who enter the status due to a specific long-standing bad thought
Succubus/Incubus: A creature that arrives at your fortress appearing to be a regular dwarf migrant. The foocubus obtains relationships faster than normal, and tends to slack off work in order to socialize. When the foocubus obtains a lover, that dwarf's card is marked, and about when their relationship would be sufficiently developed to marry, the partner dies. Unlike normal dwarves, the foocubus forgets its lover-victim immediately and starts forming new relationships. Dwarves who become suspicious (and to a lesser extent those with suspicious friends) will avoid the creature and enough suspicion will result in it being unmasked and trying to flee the fortress like any skulking enemy that's been seen.
Doppleganger: A creature that could enter the fort looking like anything, when it finds itself alone with a dwarf or domestic animal, it will attempt to kill its victim, and then take the victim's place. If successful, dwarves won't notice the difference or get bad thoughts from the original dying, at least until the doppleganger changes shape again. A doppleganger has no particular goals or targets, it just, like most things, does not like you and has a unique way of showing it. A doppleganger might be noticed transforming, or its activities might arouse suspicion.
Saboteur: A dwarf turned against its kind (perhaps the result of goblin baby-snatching?) the Saboteur acts like an ordinary dwarf, except he or she will occasionally pull levers at random or deconstruct traps. These activities are suspicious of course, and dwarves who witness the Saboteur might have an inkling that dwarf is up to no good.