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 noble unicorn. These proud and noble creatures have a long history in fantasy media, but rarely serve as creatures that the average group of adventurers have a reason to fight. The first unicorn in this book, the sylvan guardian unicorn (CR 5), is not a paragon of good, but rather of nature, and will do anything to defend its sylvan home, especially impaling hunters and woodsmen who might threaten it. The storm unicorn (CR 8) is a mix of a unicorn and a pegasus, and possesses the ability to fly, as well as an affinity for storms and lightning, which it can fire from its jagged metallic horn. These elusive creatures are rare and exotic game, and are not noble paragons of virtue like their more common cousins. Finally, there is the purifier unicorn (CR 10), which is so pure and divine that non-good creatures that look upon it are struck blind. While it makes a suitable challenge for a group of evil adventurers who are far too powerful to find a challenge in the standard unicorn, its magic tears provide a reason for characters of any alignment to attempt to subdue one, if the need is dire.