Detritus the Troll, more so in his earlier appearances. Most Discworld trolls are Dumb Muscle, depending on the weather.
So far as the weather goes, trolls use silicon-based brains, much like computers. In warm climes, their brains overheat, and they become stupid. It's been said that while smart trolls come down from their cold, cold mountains, dumb trolls arrive on the warm, warm plains. Detritus got his intelligence upgrade when a friend figured this out and built a fan into his helmet.
Mr. Tulip, from The Truth
Terry Pratchett often subverts this trope by putting clever or profound statements in the mouths of Dumb Muscle characters, often with the smug addendum, "I may be slow, but I ain't stupid."
William de Worde: William de Worde. Ankh-Morpork Times.
Detritus: I don't read dat.
De Worde: We'll put out a large print edition.
Detritus: Ho, ho, dat very funny. But fick as I am, I'm still the one saying you can't go past.
The elder brother and classmates of William de Worde, from the quote above, are likewise described as Dumb Muscle, albeit blue-bloodedDumb Muscle who'll actually be hiring lower-class Dumb Muscle to do the Dumb Muscle work for them someday.
Averted in Conan the Barbarian: Conan is cunning, and in the original written portrayals, he is quite knowledgeable about anything within his fields of interest; military tactics both land and sea, navigation, the customs of various lands, all manner of thievery, and could read and speak multiple languages. He's just not a scholar, having learned these things through extensive travel and the school of hard knocks. Most derivative works parody him as stupid, though.
Lennie from Of Mice and Men is an archetypal example. Deconstructed in that being a mentally retarded individual with huge strength brings him and those around him nothing but misery, and ultimately leads to the infamously tragic ending where Curley's wife stupidly flirts with him, then his dim-witted fixation with petting her hair makes her panic; he's so dumb he can't realise that she's scared and so, when she screams in fear, he tries to make her be quiet only to smother her in the attempt, forcing George to shoot him to keep him out of the brutal hands of a lynchmob.
Subverted in the Dragonlance series with Caramon Majere. Although at first glance he seems like a Dumb Bruiser, content to just hit stuff and let Raistlin or Tanis do the thinking, he is quite insightful and the authorial annotations remark that although he may not be smart, he has always been very wise and capable of making good decisions if he thinks things through.
Gamorreans in the Star WarsExpanded Universe, especially Jabba the Hutt's Gamorrean Guards, who include such powerhouses as Thok, the incredibly appropriately named Thug ("When you need muscle for a job, a Gamorrean makes a good choice"), and Gartogg, who was dim even by Gamorrean standards, having Hulk Speak while others do not.
A Song of Ice and Fire: Hodor, the retarded giant, and Small Paul, the simple-minded soldier. Other big and strong characters tend to display less than exceptional intelligence, and the race of actual Giants in the series is noticeably low-tech. On the flip side, the series's smartest characters, Petyr "Littlefinger" Baelish and Tyrion the Imp, are quite short.
Subversion: This is how most characters in L.A. Confidential perceive Bud White. Dudley Smith ostensibly drafted him into his enterprise based on his scary looks and propensity for violence. Turns out, however, that he is both smarter and more noble than anyone (himself included) gives him credit for.
In the third Artemis Fowl book, Mulch is taken away by two of these, Pex and Chips. They were not hired for their brains, and are not allowed out of Chicago, "as this could involve map reading." When they're taking Mulch away, their conversation is them figuring out the other's nicknames-after much discussion, they figure out that one has huge chest muscles, and the other eats-oh, never mind.
Crabbe and Goyle in Harry Potter are thicker than trolls and built about the same way - they spend most of their time flanking Draco Malfoy and hardly even speak. On the Muggle side, Dudley Dursley (Harry's cousin and childhood bully) is mostly "dumb blubber", though his sheer size is enough to intimidate most kids his age. He gets worse after taking up boxing, though Harry himself is used to things much scarier than Dudley by then.
Also trolls themselves, as well as giants (typified by Hagrid's half-brother Grawp).
The Honorverse has the Scrags (formally, Sacred Brotherhood), a decayed group of Super Soldiers that, as of the present, are rarely used as much else. The Amazons (Scrags who didn't care for the whole "women are slaves to do with as men please" thing of Masadan theology) attached to Thandi Palane, in Crown of Slaves, though, were put through schooling to correct their educational deficit, though socially they're still not quite up to speed on "subhuman" social skills.
Adus, The Brute in The Elenium, is a barely literate thug who struggles to say any word with more than one syllable, eats raw meat, spurs his horses to death on a regular basis, and communicates with his Mooks through kicks, slaps, and grunts. He is something of a savant when it comes to small unit tactics, ably deploying his Zemochs in pairs and trios to fend off the ChurchKnights despite his lack of communication skills, but in the words of the author is, in "most other respects profoundly, even frighteningly stupid."
The independent, overmuscled thug known as Tony Donuts in "Callahan's Lady," by Spider Robinson. Tony is actually so stupid that he's difficult to con; against him, a character warns, a plan has to be not just foolproof, but moron-proof. This comes to a head when he demands his counterfeit money back and a con artist tries to give him real bills; the trouble is that all of Tony's bogus bills have the same serial number — it was too much trouble to change the plates — so that he instantly recognized he was being tricked.
Your greatest error was in striving at first for such physical perfection. You have overdone it, with the result that the court of mystery is peopled by a dozen brutes of awful muscularity, and scarcely enough brain among the dozen to equip three properly.
Nicko Heap in Septimus Heap is described as being unusually strong but also often as careless.
In terms of the Achaian warriors of the Trojan War, Ajax Telamon is this. He is the largest and strongest fighter in the war, and the Achaian's second best fighter after Achilles. He is also noted for being less cunning and eloquent than the other big name heroes. His downfall comes when Odysseus defeats him in a debate on who should get Achilles' armor. The stories differ, but all agree that he ultimately kills himself in shame.
Br'er Bear in the Uncle Remus' stories.
Shota from Paladin of Shadows. He has difficulty counting to five, but wears ridiculous amounts of armour and carries a rocket launcher like the other Keldara carry rifles.
Menelaos in Greek Ninja sometimes appears dumb, although he's more of a goofball rather than actually dumb.
In Firebird (Lackey), all of Ivan's sons, save for Ilya, are Dumb Muscle to varying degrees. Piotr however is the most cunning... not smart, but cunning. The Katschei's demons are even worse.
Almost always averted for human beings in The Laundry Series; you have stupid zombies, demons, etc. but even the random human guard or mook is actually smart. The OCCULUS team, which is The Cavalry (a Badass Normal elite military unit specialized in dealing with occult threats), are both expert fighters, well-organised soldiers and very intelligent people in their own right. Justified, because the books make it very clear that people involved in occult intelligence or in sorcerous schemes to take over the world just can't be dumb if they expect to survive a few weeks of work, whatever their level of badass.