(So much more than just another bloke in a dress.)
An Example: The Hunting Beast (CR 13)
You can change the athach to any type of creature you wish that challenges the PCs and is appropriate for the terrain it is encountered in.
The deep pines sag about you as you continue your journey, The air is still here, almost suffocating, and the shadows within give little hint of the daylight visible far above.
Allow the PCs to journey on their way through the forest, occasionally making the odd dice check to keep them on their toes, before suddenly six figures dart through the gloom ahead, heading straight for the PCs.
Creatures: A herd of dire boar is dashing out of the forest and away from the athach, which is hunting them and has already slain one of their kind. The boars are encountered 240 ft. from the PCs and run straight toward them.
Dire Boar (6) CR 4 each
XP 1,200 each
hp 42 each (Pathfinder Bestiary 36)
Tactics: If left alone, the boars not only approach the PCs at full speed, but actually rush past them, only attacking if they themselves are attacked, in which case, petrified that they have been surrounded, they attack until slain.
Athach CR 12
hp 161 (Pathfinder Bestiary 2 33)
Tactics: The athach, Gnarlrudd, is after good cruel sport, and it is ambling after the boars, happy that he can catch them and not only kill them horribly to eat, but save their hides for his beloved wife Harpywide, who lives back in their vast timber lodge along with their children Crush, Spite, and Menace. Maybe if he gets lucky he’ll take one alive for his offspring to play with later. If he finds that someone else is trying to take his sport from him, however, he’ll make that creature pay. Gnarlrudd thrives upon misery, and he happily watches anyone attack the boars, thinking that, once weakened, the sport is going to be all the tastier. If the PCs engage the boars, he lurks back watching, hoping to pick the odd stray character up. If he overpowers a single PC, he runs with them back to his vile home about three miles away, where he prepares something particularly uncomfortable for his victim involving an oven, a pair of iron boots, and a wire brush.
Once discovered, the athach attempts to take a single PC back as a gift for his lovely, fleeing combat with any unconscious characters he can.
The nearby lodge is not only huge—it is more like a castle than a home. Indeed, if you wish, it could be an old castle. The attack upon the home should present a serious challenge to your PCs, any friend of whom is kept in a suitably unpleasant situation (say, suspended over a boiling cauldron of water in case his or her friends foolishly try to rescue them).
Treasure: Gnarlrudd has a silver bound hunting horn made from the horn of a dire bull worth 250 gp, he carries enough dried meat to last him two weeks, and he has a huge twelve pint water skin as big as a man. The latter is full of fresh spring water. His hefty coat is trimmed in wolverine fur and has buttons made from platinum coins (there are twenty in all); as a curious object in itself, the coat is worth 350 gp. The coat has dozens of pockets, butcher’s hooks for game (with three sheep hung on one), and folds, within which are a potion of invisibility with a green glass stopper, a chewing stick made from a tree-trunk, a silver penny whistle worth 5 gp, and a Small halfling cloak made of grouse feathers, with silver thread worth 125 gp.