Exotic Encounters: Centaurs
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 centaur. These icons of Greek mythology have fulfilled many roles in fantasy media, from savage barbarian stand-ins for Mongols, Huns, and other nomadic horselords, to fey-like guardians of nature and wilderness, with many roles in between. In Pathfinder, their lowly CR 3 status and the fact that their horselike bodies don't really belong in underground dungeons means that many adventurers may never encounter one, but the new centaurs in this book may just change all that. The first such centaur, the plainsrunner centaur (CR 4) ties into the image of centaurs as steppe-dwelling nomads, and these centaurs are exceptionally fast and agile, making for incredibly maneuverable opponents. The second centaur in the book, the skyborn centaur (CR 6), is partially celestial in nature, and has great white wings that allow it to fly through the air. It specializes in jousting and duels, and is incredibly proficient at charging with a lance. Finally, the nightcharger centaur (CR 8) is the spawn of a night hag and a nightmare, and combines their most dreaded powers in new and unusual ways, as well as having its own curse of doom ability, which blocks its victims from having any lucky breaks, and exacerbates the effects of bad luck.