The Dinobots from Transformers. People say Grimlock is smarter than he lets on, but he still wants Kup to tell him about the petro-rabbits, even in the middle of a firefight. Sludge is also particularly dim, and, as the largest of the Dinobots, particularly strong. All of the Dinobots have an idiosyncratic form of Hulk Speak that lends itself to this image.
In later issues of the comics this is subverted. Depending on the writers, the Dinobots are either street-savvy bruisers, or regular Autobots who feign ignorance as an act. Grimlock is consistently portrayed as being a temperamental leader who prefers direct action.
In Transformers Animated, Grimlock is almost toddler-like in his temperament and speech; Swoop and Snarl don't seem to have enough processor power to talk at all, either.
Animated also includes Lugnut, who is delusionally devoted to Megatron, Mixmaster and Scrapper, who are barely aware of their surroundings, and Blackout, the largest Decepticon in history, who needs to be closely supervised in order to be of any use.
It's not so much that Lugnut is dumb, he's just blinded by his extreme loyalty. He has his moments, like "Decepticon Air" when he led a small band of Decepticons in taking over the Elite Guard flagship.
Also, in Generation 1, most combined robots were extremely crude mentally. Menasor was decidedly Hulk-like, for instance, only being able to act on things all of his component robots could agree on. Superion, similarly, had exactly two things he could do or think about: Fight and Protect. Even the super-smart Computron took so much time processing data that he was useless in battle. Motormaster and most of the original Predacons, as well, especially the hardheaded Headstrong, the easily enraged Tantrum, and he hyperactive Rampage. Bruticus and Abominus are even more pronounced examples, to the point where they aren't ordered to fight as much as turned loose to engage in wanton destruction. And then there's Beastbox, an interrogation expert who is often faced with extracting information from an Autobot he had beaten to death seconds before.
Outside of the cartoon, the true animal combiners (Predaking and Piranicon) subvert this. The explanation is that the component robots have a shared love of the hunt that completes the persona of the combined form.
What the Firecons lack in intelligence and direct physical power, they make up for in, ahem, firepower. The exception is their commander, Sparkstalker, a twisted genius wasting away babysitting moronic stooges Flamefeather and Cindersaur.
And Skullcruncher, who is specifically described as "slow (in more ways than one) but powerful." This is the guy who eats his enemies after defeating them, even though Cybertronians don't get nutrition of any kind that way and he admits to hating how they taste. However, the surprisingly sophisticated symbolism of cruelly absorbing his foes isn't lost on Skullcruncher's Headmaster partner Grax, who can do enough thinking for the both of them easily.
And Stranglehold, whose strength has always outweighed his intelligence by a hefty margin. While he's a genius fighter, he's a moron when it comes to everything else, making him ultimately a liability and only welcome on the Mayhem Attack Squad, a team of misfits with similarly bad reputations.
And Tidal Wave from Armada, who is basically only capable of saying his own name, and Demolishor's later incarnation in Energon.
Crumplezone from Cybertron. With his big forearms and short legs, he also looks like an ape. He's literally the largest mechanoid on Velocitron. And while compared to the rest of the natives he's very slow in a couple of different ways, he's much quicker than his size suggests.
BB from Beast Wars II subverts this slightly; His alternate mode is a stealth bomber. A gigantic stealth bomber equipped with a similarly enormous gatling gun. He's roughly three times as wide as your average transformer in robot mode, has strength and firepower ratings of 9 and an intelligence rating of 4, and can only say "Roger!"
Blastcharge in Beast Machines, and Tankor, until Rattrap roots around in his head.
In the Shattered GlassMirror Universe, Buster and Spike Witwicky are the dumb muscle of the family gang, while oldest brother Butch is the brains. The SG Technobots are also in the "thick as bricks" category, thanks to a failed experiment that rendered them neurotic in their individual forms and arrogantly stupid in their combined form.
Wild Kratts has Davio, henchman to Donita. Leads to a good Brick Joke in Fireflies when Donita tells a joke at the beginning of the episode, and then Davio gets it in the end... right when Donita is too angry about being foiled again to care about it any less.
Davio: "Glowrious! I get it! Haha, very funny Donita. LOL." Donita: "You don't say LOL, you just laugh. AUGH!"
Not to mention Humongoloid, whose tremendous size comes at the apparent cost of having Down's Syndrome. At least until he gets shrunk, at which point he displays average intelligence.
Hummongoloid wasn't developmentally disabled, he had gigantism. His slow, slurred speech (pre-shrink) was simply a result of his tongue being too big to manage.
Mr. Larrity's son, Dean, from Code Monkeys. Though he's strong, he has the same type of mental weaknesses played for laughs. "I love Japan! Except for their Reggae music, cause it has the word 'gay' in it."
Kronk from The Emperor's New Groove. However, he is an excellent chef, and he does occasionally bring out closer to Earth insight.
The Infraggable Crunk (and actually Val Halen and Major Glory as well, to some extent) from the Justice Friends, the shorts from Dexter's Laboratory. Incredibly strong, but with the psyche of a 5-year old... literally. "TV Puppet Pals! TV Puppet Pals! Lots of fun for boys and gals!"
And Crunk's Distaff Counterpart, the supervillain She-Thing. She promises to write every day from prison, prompting Crunk to promise to learn how to read.
Several in He-Man and the Masters of the Universe (2002). Ram Man makes an impressive showing for the good guys, but he's eclipsed easily by Clawful, whose voice alone comes on a little too strong about the fact that he's an utter moron. (He's so dumb that in one episode, he can't understand the non-verbal language of his own species.) As it happens, in the original series, he was actually a Genius Bruiser, a role that was taken by Tri-Klops... who was Dumb Muscle in the original show, himself. Then there are guys like Beast Man and Grizzlor, who are frankly just bestial and have a lot of know-how related to their skills, and Spikor, who manages to be a Small Name, Big EgoLarge Ham who strikes poses while boasting about his imminent victory in addition to being an idiot.
Numbah Four of the Kids Next Door is the idiot of the team, but also the most inclined to physical violence. His actual combat ability varies greatly between episodes, usually switching between Rule of Cool and Rule of Funny.
In Gargoyles, Broadway's clone Hollywood (and, indeed, the Clone Clan as a whole) is a straight example of this trope. Broadway himself at times seems like this compared to Brooklyn and Lexington, but he's too smart to be a completely straight example.
Mammoth, Cinderblock, Overload, and Soto in Teen Titans.
Subverted with Cyborg and Monsieur Mallah, who use brute force the most of anyone on their respective teams, but are both highly intelligent as well.
Torvald and Harold in Hey Arnold!, who are both 13-year old fourth graders as a result of being held back several times. The best grade Torvald ever had in his life was a C- from kindergarten.
Subverted with Red Menace from League of Super Evil. He's shown to be of normal intelligence and quite resourceful at times. In one episode, he even scored higher on a computerized intelligence test than Voltar (whose intelligence was on the level of celery).
Doctor Frogg became one after using a device to convert his brain power to strength, until they needed to disarm a self-destruct mechanism and needed someone smart to do it (when Red needed to "re-educate" him).
The Bullnerds, Thor Thort and Brick Buster in ChalkZone.
Susan Strong, in Adventure Time, is almost a rare female example, but actually could be considered a subversion. She can't speak English right off the bat and is very confused about life on the surface, but that doesn't make her dumb; she learns very quickly.