The Pathfinder Bestiary, and the long line of monstrous encyclopedia that came before it, do a great job of attacking the problem of providing game statistics and mechanical information for a very wide variety of creatures. There are a lot of strange creatures from fantasy novels and movies, from ancient mythology, and, as time went on, from the twisted minds of game designers and even GMs, for that matter. Anyone today who tried to go about creating an extensive bestiary of all the different cool and interesting fantasy monsters could fill three or four volumes with three hundred monsters each, and still have a long way left to go, with fans crying out for various "forgotten" monsters to get some attention. That kind of breadth doesn't leave a whole lot of room for depth, and that's where Exotic Encounters comes in.
There are all kinds of reasons why you might not want to use a monster stat-block straight from the core rules. For one thing, if your players have a habit of browsing through such books, or are long-time veterans who know the basic ins and outs of most fantasy staples, you may need an unusual stat-block just to throw them a curve ball and teach them that they can't count on their out of character knowledge to take all the mystery out of the game. Alternatively, many GMs can recall a time when they wanted to make use of a certain monster in their game, only to find that it was a few CRs too high or low for what they had in mind.
Exotic Encounters takes a single, iconic monster, and creates three new variants on that theme. These variants aren't simple tweaks, and are more than simply advancing or removing Hit Dice, though that occurs as well. Each of these three variations on the monster's theme has a specific, flavorful goal in mind, and a role to play, and their statistics entries are gently massaged in order to make them fit those roles. Further, each of these variants comes with brand new specially-crafted special abilities, which are unique to Exotic Encounters and not found anywhere else.
This particular installment of Exotic Encounters focuses on the couatl, a creature that is exotic by almost any definition of the word. From their South American background (which stands out like a sore thumb in the medieval-European-centric world of fantasy roleplaying games), to the fact that few parties have any reason to ever fight one, it's a fair bet that your game group has never encountered a couatl before. This book aims to provide some couatls that might actually see some use, the first of which is the jet couatl (CR 11), a type of couatl that has fallen from grace and forsaken its heavenly background, as well as its wings, in favor of a variety of new profane powers. Also featured is the primal couatl (CR 13), which is more in tune with the forces of nature than the heavens, and possesses a powerful hypnotic gaze and hallucinogenic venom. Finally, the radiant couatl (CR15) is a paragon of the typical couatl's ideals, but in addition to being advanced and having better spellcasting, its radiant wings can disable those who gaze upon it, and its blood is a fabled cure-all, giving characters of any stripe a reason to come to blows with one.