Just in case anyone missed it in the opening paragraphs, Fat Bastard from the Austin Powers films is the Trope Namer. He is an ill-tempered Scot who weighs a metric ton and Eats Babies. Though the end of his debut movie does try to paint him in a sympathetic light for about a minute or two...and then he blows it by farting at the end of his heartfelt monologue, and decides to kill Austin and his Love Interest anyway.
Anyone and everyone played by Sidney Greenstreet, though they usually softened the blow by being Affably Evil. The most famous is Kasper Gutman (no, really) in The Maltese Falcon.
Bonus points as Gutman is called "The Fat Man" by the characters.
Most of the characters played by Laird Cregar, perhaps most memorably the prissy chocoholic villain Willard Gates in This Gun for Hire.
The Ten Commandments: Jannes the High Priest of Egypt, played by Douglas Dumbrill, and his frustration at Moses' action in taking away his and his priests' temple grain to feed the Hebrew slaves leads to his secretly siding with Moses' rival Rameses.
Jurassic Park: Dennis Nedry is slobby, he's abrasive, he's played by Wayne Knight, and he's a Corporate Spy who sabotages the entire island to steal some dinosaur embryos for InGen's rival, BioSyn. He also meets a tremendously Karmic Death at the claws and poisonous spit of a Dilophosaurus.
Jabba the Hutt of Star Wars fame. Yes, he's an alien, but he was originally supposed to just be a fat hairy alien before Lucas's budget got boosted for the two sequels.
Not to mention that he is fat, even for Hutt standards.
Wesley: She has one single iota of tenuous power. She thinks she can push everyone around. (Grabs Janice's stapler) You don't need this. I understand. Junior high must've been kind of tough, but it doesn't give you the right to treat your workers like horseshit, Janice. I know we laugh at you, Janice. We all know you keep a stash of jelly donuts in the top drawer of your desk. But I want you to know, if you weren't such a bitch, we'd feel sorry for you. I do feel sorry for you. But as it stands, the way you behave - I feel I can speak for the entire office when I tell you... go fuck yourself.
Zigzagged with Jelly, Paul Vitti's right-hand man in Analyze This. He is fat (likely how he got his name), he is a mobster (and he has killed a few people in his career), but he's more of a Noble Demon.
The Commitments about an Irish R&B group has lead singer Deco an obnoxious and egomaniacal lout.
Mr. Creosote from Monty Python's The Meaning of Life is so fat that he can hardly walk, and is very rude and inconsiderate to the restaurant staff. The worst part is that he continuously vomits on everything, including the floor, the menu, the cleaning lady, and the maître d', while making everyone else's dining experience so unpleasant that there's a Vomit Chain Reaction.
Cross-dressing John Waters star Divine, playing a man, was always unsympathetic and unlikeable. As a woman, at least "she" was the protagonist (extended to non-Waters movies like Trouble In Mind and Rustlers' Rhapsody.)
The Great Goblin in the film adaptation of The Hobbit.
The dead-o Pulaski, a.k.a. "Fat Elvis" from R.I.P.D.. Applies to a few others, as well.
O'Brien, the local rep for the Southern Oil Company in The Wages of Fear, who happily sends the central characters on what is essentially a suicide mission for little more than the price of a ticket home.
The film versions of Harry Potter take it to extremes with Vernon Dursley.