You won't find two of the best Pathfinder monsters in the Bestiaries. The terrifying charau-ka (from Pathfinder Adventure Path #40 and The Inner Sea World Guide) and the straight-out-of-a-pulp-novel angazhani or high girallon (from Heart of the Jungle) brachiated their way into other books. Thankfully, the baregara is still howling through the pages of the Bestiary 3.
Falling just short of true demon status, baregaras are the apes of the Abyss, moving in troops and spreading terror throughout the evil plane’s jungles. (A baregara’s monstrous challenge, the ability to grapple with one arm, and the giant mouth in the center of its chest to bites whatever it’s grappling helps with the whole terror thing.) They don't have much in the way in magic, but what they do have allows them to get the drop on PCs quite effectively, with a barrel of dire apes or girallons allies to boot. (See what I did there?)
And let’s be clear—you don't have to wait till PCs reach the Abyss to deploy these things. There are plenty of savage spellcasters happy to summon these creatures. (You can feel free to fudge level limits as well—NPCs have access to all kinds of rites, rituals, and artifacts PCs don’t, especially when they live in a blood-soaked temple forgotten by time.) And plenty more baregaras have probably found their way to the Material Plane of their own accord. Zoos can’t even keep orangutans in their cages, so what do you expect from demon orangutans?
(Murder. You can expect murder.)
I should check myself before I leave the impression that these creatures are brutes, though. Wild, sure. But at Int 15 and Wis 16, they’re smarter than even the armor-wearing high girallons, and even without telepathy they speak four languages, including Draconic. So imagine Planet of the Apes. Now imagine Zira, Cornelius, and Dr. Zaius as demons. The movie would have been a lot shorter…
A crime lord has seized control of the sumo wrestling circuit. Those wrestlers who do not submit to the fixed matches and choreographed fights find themselves “invited” to more private matches—against the crime lord’s pet baregara.
A baregara troop attacks a party of adventurers…and then pulls back. The adventurers apparently have a memento that marks them as untouchable. Still the baregara troop shadows them for miles, occasionally testing the party with summoned girallons or unholy blight effects. Eventually, their greed for the trophy gets the better of them and they attack.
“There…has been an insult.” So says the red dragon currently demanding an adventuring party’s help. “I cannot act. But you can.” A baregara managed to abduct her wyrmlings, blackmailing the dragon into surrendering her hoard and her mountain hold full of slaves. If the adventurers rescue her children, the red dragon will be in their debt—assuming she honors her word.
—Pathfinder Bestiary 3 34
I think apes are creepy—clearly an uncanny valley response—so I highly approve of demon-apes as villains.
And 3.5 players, don’t feel left out—these adventure seeds will likely work just as well for the bar-lgura from Book of Vile Darkness and Hordes of the Abyss.