The Big Kahuna
Demogorgon: Pictured above, Demogorgon is the self-styled "Prince of Demons" who keeps the title by sheer virtue (vice?) badassitude. He is one of the "Big Three" archfiends of D&D, joined by his fellow Eldritch Wizardry alumni Orcus and AD&D's Graz'zt. Despite his baboon/mandrill heads, his only worshipers in older editions (circa AD&D and earlier) were very rarely evil Humans and most commonly Ixitxachitl, sapient evil manta rays with occasional vampirism thrown in. During this time period he also had a strong vampiric theme (because of the Ixitxachitl) which was later dropped and he once changed sex for no discernible reason in the Immortals boxed set. The 3rd edition of D&D added Lizardfolk and Yuan-ti (evil snake people) to his list of followers, then obliquely hinted that he has a jungle of apes and monkeys in his home realm but never actually bothered to talk about them or really detail any simian cults of Demogorgon. The only instance I could find was the Dungeon #120 adventure "The Lost Temple of Demogorgon" written by Sean K. Reynolds; in features several awakened (magically sapient) gorillas, giant gorillas, a baboon sorcerer, and a baboon priest. I'm utterly at a loss as to why D&D keeps focusing on his amphbious/reptilian qualities while ignoring his simian qualities...
One last note on Demogorgon is that his two heads have different personalities: Aameul (right) is suave and cunning while Hethradiah (left) is savage and simplistic. Both heads hate each other and are constantly scheming on how to separate/kill the other without dying in the process.
Smaller, but still rather Large, Kahunas
Angazhan: A huge, horned gorilla with a tail who rules over a horde of vicious demon apes in the Abyss while his cult thrives in the deep jungles of the mortal world. He's actually from Pathfinder, but PF is just a D&D clone so it seems fitting to mention him. He also the king of the Baregara and several mortal ape-like races.
Fraz-Urb'luu/Var-Az-Hloo: Not technically an ape but "ape-like", Frazzy is the demon prince of deception, which for some reason means that he could summon other demon princes willy-nilly to his castle to humiliate them. Tired of his shit, several demon lords gave a crazy Human wizard some kinda magic sword and he whupped Frazzy's ass and sealed him in a bas-relief on a dungeon wall. He later escaped thanks to dumbass adventurers and now hates Humans. He's traditionally an enemy of Demogorgon.
Ilsidahur: A huge, horned gorilla with a tail who rules over a horde of vicious demon apes in the Abyss while his cult thrives in the deep jungles of the mortal world... In case you didn't notice, Pathfinder basically ripped off Ilshiadur to make Angazhan. They're even both lords of similar-sounding demon types (Bar-Lgura and Baregara). One thing that distinguishes him from Angazhan is that in his original Dungeon incarnation he also has necromantic powers that can only be used on apes. He also has a greedy streak and an eye for bronze.
Mandrillagon: Demogorgon's "brother" from Gary Gygax's Gord the Rogue novels is technically speaking not a D&D entity since TSR rendered Gygax's books non-canon when they fired him. He's basically a giant ape with a blue mandrill face. He really seems like a huge pansy despite his huge stature. HE'S UNABLE TO CONTROL HIS SPEECH VOLUME SO HE'S ALWAYS YELLING. He also secretly hates Demogorgon.
Bar-lgura: Pronounced "Bar-le-goo-ra" because Gygax was whacky like that, these demons started out as six-fingered, sinister orangutans before turning into something closer to a musclebound gorilla with tusks. They are canonically the thralls of Ilshiadur but since he's a z-list demon lord Demogorgon tends to have more of them. The Barry-Gra are also telepathic.
Baregara: From Pathfinder, these giant horned gorilla-demons have an extra mouth on their chests for maximum rip n' tear potential. They are thralls of Angazhan. Seem like ideal shock troopers/scouts for a simian demon army.
Nalfenshnee: Giant ape-pig-demon things with tiny wings, the Nalfenshnee are the judges of the Abyss...don't ask me how beings of undiluted malevolent chaos can have a judicial system because I don't have an answer. Compared to the two BGRAs above, the Nalfs are a bit more magical even if they do pack a physical punch. They're also fat.