Aladdin: The Series: Destane, the previous ruler of the Land of the Black Sand before Mozenrath usurped him, was apparently so vicious and powerful that Jafar himself steered clear of him.
Megatron is this in Transformers Animated. The Autobots regard him as a bogeyman who is reputed to eat protoforms - baby Transformers. The other Decepticons are viewed this way as well. The Autobot leaders are so afraid of them that Ultra Magnus and Sentinel Prime would rather deny the Decepticon presence on Earth than cause a panic among the populace.
Soundwave in Transformers Prime is this to the other Decepticons due to his role as communications chief, which helps him keep down insurrections.
Don't let his slow moving nature and silence fool you, Soundwave is fully capable of beating insubornates within an inch of their lives. Airachnid learned this first hand.
General Grievous in Star Wars: Clone Wars. His near-victory against five (albeit exhausted) Jedi established him as a fearsome enemy. Count Dooku actually taught Grievous to use fear as a weapon, telling him that without it he wouldn't stand a chance against the best of the Jedi.
Zecora is this to the population of Ponyville at the start of "Bridle Gossip," who believe her to be an evil enchantress thanks to her strange mannerisms and residency in the dreaded EverfreeForest. She's revealed to be perfectly nice by the end of the episode.
Breach from Generator Rex becomes this in "Lions and Lambs". Everyone who knows what Breach can do fears her. When Six notices the Providence troops are trembling, he asks Rex if they have any experience with Breach. Rex says that's the problem, shuddering as he remembers what Breach did to him.
Almost every member of the Kids Next Door is terrified of Numbuh 86, and not just because she has a foul temper; she has the authority to have them decommissioned. (That's her job.) Of course, she quickly becomes a Miles Gloriosus when faced with a problem she can't handle.
Princess Azula from Avatar: The Last Airbender . Being a Machiavellian tactical genius, she deliberately goes out of her way to solidify this image of herself, believing in manipulation through fear and might. For much of the series, it works spectacularly, with even her Uncle Iroh showing fear of being in her clutches, and leading to her turning an entire Government Conspiracy group against their leader through sheer intimidation and force of personality alone. Unfortunately for her, it stops working when Mai's love for Zuko, and Ty Lee's friendship with Mai, become stronger than their fear of her.
In The Legend of Korra, Amon the leader of the Anti-bending Equalists is this, thanks to a combination of his creepy masked appearance, his even creepier (and very competent) minions, and his ability to permanently remove a person's bending. Korra is terrified of him and sees him in her nightmares.
In Jimmy Two-Shoes, Lucius I-the original ruler of Miseryville-is this. He's never actually been shown on the show, but he was mentioned in one episode, and it was shown that all the rest of the Heinouses (all of whom are basically different incarnations of Satan) are horrified by him.
In a universe filled with Eldritch Abominations, Evil Sorcerers, Alien Invaders with Death Rays available on a large scale, Ghost Aliens Evil Overlords, Monster Clowns and psychotic mutants amongst other things, Vilgax is still considered the most dangerous being.
Even Chimera Sui Generis are afraid of him, and he's one of them.
Total Drama All-Stars has Mal, one of Mike's personalities whom all the other personalities (Chester, Svetlana, Vito and Manitoba Smith), and Duncan fear. In "Evil Dread", as his other personalities talk about him returning after years of absence, Svetlana is clearly terrified and Vito worries about how they can stop him. Chris, Chef, and Alejandro are mortified of him as well.
In the original Bugs Bunny shorts, the Tasmanian Devil was this. Every single animal in the area would panic and clear out when he started approaching.
Ben 10: In the original series, Vilgax, being an Evil Overlord who has conquered ten planets, destroyed five others and created a black hole, is so feared all over the universe that no organic life form is willing to work for him, forcing him to use Mecha-Mooks as his personal army.