There is dozens of "sneaky" monsters (Chokers, Leonels, pixies, invisible stalkers, most Demons have hide ranks, etc.), many of which use "shady" tactics (Shadow's in-ground-move-attack-move-back-to-ground-haha-can't-attack-me), but I really can't think of any that out right gain a bonus if the opponent is Flat-Footed or w/e. Well, there is an Epic monster I know of...
Most monsters have loot, give them a SA granting item.
DM edit in Sneak Attack to the creature.
Or, "Tyranids" are DarkToM HellbredFCII Beguilers and/or Shadow Hand specialized Swordsages, renamed and reflavored of course.
One of their champions can be a Hellbred Carmendine Dark Moon Disciple Monk 7 / Unarmed Swordsage X, they get Total-Concealment only bypassed by Blindsight-like effects and THE SUN.