Big Bad Pet

A Giant Mook that's usually an animal, or any non-human (commonly referred to as a "creature") for that matter, who is kept by the Big Bad as a pet.

Often too large or wild to be in the Big Bad's main quarters, the Pet is usually kept in a cage, cavern, dungeon, or any other secluded place. The Big Bad Pet's job is usually to guard the Big Bad's lair, or to viciously consume the hero once the latter is thrown in a pit of death. It's usually too dumb to actually be given orders, so it sort of serves as a Personal Mook for the villain.

To elaborate, Big Bad Pets are, well, big and bad. They usually are composed of dragons, snakes, sea monsters, giants, lions, bears, big hungry dogs, sharks, giant man-eating plants, and the like. They're the creature version of The Dragon, only usually much larger and of savage nature. Thus, they usually don't get that much character other than growling about to frighten the protagonists.

Examples of Big Bad Pet include:

Anime and Manga

Caesar Clown, the Arc Villain of One Piece's Punk Hazard arc has one in the form of Smiley, a mountain-sized mass of poison that ate the Sala-Sala Fruit: Model Axolotl. The result is a monstrous salamander kaiju made entirely of poison that Caesar uses as a glorified attack dog. While he can order it to follow basic instructions, it'll ignore him if it's interested in something else (like candy) and is generally so mindlessly destructive that it'll kill his men for getting in the way. Not that Caesar minds, given the kind of person that he is.

Comic Books

Issue 9 of I Hate Fairyland has the antagonist Bart of the Blackness who threatens to feed Gert to his large and apparently tame pet snake Mr. Slitherington, who appears more than happy with this arrangement.

Film

Jabba the Hutt kept a pet rancor in Return of the Jedi. (Wookieepedia says that its name was Pateesa.)

Literature

Bill Sikes kept his attack-dog Bull's-eye by his side for most of Oliver Twist. Bull's-eye was shown to be vicious, and helped add to Sikes' menacing nature.
In Sherlock Holmes, this happened a few times:
- "The Speckled Band" is revealed to be a snake that was trained to attack anyone sleeping in a particular room that was connected to Blackwood's chamber. Blackwood used the snake to kill one daughter and planned to murder another. Sherlock used a whip to send the snake into a panic and back down the passage, where it bites and kills Blackwood instead.
- The Hound of the Baskervilles reveals that the titular hound is this. Not a ghost, not a monster, but a hound that was trained to hunt down people in the moors. Once Watson shoots it, he and Holmes realize that the Big Bad Stapleton coated the dog in phosphorus to make the poor canine look scarier in the dark.
Ray Bradbury paid homage to Sherlock with the Hound in Fahrenheit 451, a programmable robotic attack dog that is a Super-Persistent Predator when set to hunt down a fugitive. While Montag is lucky to escape the Hound alive, the readers in the forest reveal to him that the Hound is trained to go after a random passersby instead and pass them off as the fugitive in question.
Shelob in The Lord of the Rings is treated like this by Sauron and his Orcs, who are content to let her roam around Cirith Ungol since she attacks and devours anyone unfortunate enough to catch her attention. However, Gollum alludes to talking with her, meaning that she's most definitely an intelligent being rather than a mindless monster. She is, after all, no "ordinary" giant spider - she's the daughter of a bonafide Eldritch Abomination!

Video Games

Recurring Mario villain Petey Piranha is used as one by the Shroobs in Mario and Luigi Partners In Time. While intelligent enough to race go-karts and play sports in spinoff games, he's portrayed as a mindless monster kept in a desert arena not unlike a lion in a Roman coliseum. Princess Peach is fed to him by Princess Shroob as part of her gambit to invade the Mushroom Kingdom of the present.
In Batman: Arkham City, Penguin has a massive, ferocious great white shark named Tiny patrolling the icy waters of his "Torture Chamber". Feeding people to Tiny is one of his favorite ways to get rid of his enemies, and Batman has to dodge the hungry shark and rescue several policemen who were offered up as a snack to him.

Western Animation

Dot isn't a villain in Animaniacs, but she does have a pet that is a giant creepy monster. It freaks out Ludwig Van Beethoven and a Jerkass on a plane who accidentally asks for "monster" when she comes by with drinks for first class and offers "Coffee, tea, monster?" without clarification.
In the Pinky and The Brain Star Wars parody featuring all of the Animaniacs crew, Flabby as a stand-in for Jabba has a monster in a pit. It ends up being Baloney the dinosaur when Yakko and Chicken Boo as Yak Soho and Chewbooboo shoot their way out after failing to deliver pizza on time. Anyone who encounters Baloney screams in terror.
In Wander Over Yonder, the karaoke bar owner in "Girls' Night Out" has one in a bit, in a homage to Star Wars. He threatens to feed Sylvia and a disguised Dominator to the beast as compensation for the damage they caused to his establishment, but in a twist, Dominator tosses him into the pit instead and runs for it when Sylvia won't go with her. Sylvia risks her life to save the bar owner. Wander ends up saving them because the kids he was escorting across a busy street were the bar owner's children, and they led him home.
He-Man and the Masters of the Universe; Skeletor has Panthor, a pet panther with purple skin. Panthor didn't seem to like Skeletor's other henchmen much and would often snarl or roar at them.
Parodied in the James Bond spell in Sabrina the Animated Series; instead of a Big Bad human with a cat, Spooky For the Lulz makes Salem the Big Bad with a tiny cat-sized human whom Sabrina and Harvey in spy form have to stop. When Salem sics his pet on them, Harvey simply traps the small person in a bucket.
In the Batman Beyond episode "Ace in the Hole", Terry investigates a criminal called Boxer who is running Beastly Bloodsports with dogs, using illegal steroidal compounds on the animals to make them larger and more vicious. When he finds the Boxer's lair, the regular specimens are only slightly bigger, but the prototype - which Boxer claims he "went overboard" on - is a hideous mutation the size of an elephant that Boxer siccs on Terry.

Real Life

Creatures used in the Roman circus generally fit this bill according to Listverse, as they were used in public executions. Hungry lions, that were trained to eat human flesh, would go after the condemned prisoners within the arena. One particularly cruel form of entertainment was strapping people naked to a seesaw, and the lions would go after anyone still unlucky to be on the end that was on the ground.
Allegedly, US president John Quincy Adams kept a pet alligator in the White House bathroom for this purpose - albeit downplayed in the sense that he didn't have it kill and eat guests. Instead, he used it to prank his guests by scaring the ever-loving hell out of them. Unfortunately, articles like this one have pointed out that this fun fact is likely a tall tale due to the lack of proof surrounding the alleged pet's existence.