And in that same alternate future, the Earth's poles shifted, somehow causing Toad and Magneto's powers to be switched. Toad took the opportunity to turn Mags into a slave for his enjoyment as payback for the mountains of abuse he'd suffered at his hand.
Ever since his first appearance in the 70s, Ras Al Ghul had a completely loyal hulking manservant named Ubu, who worshiped the ground he walked on. After a couple of apparent deaths the character was still around, so it was revealed that there's an entire Ubu clan, and they all serve Ras.
The Lowlies of Apokolips; Darkseid's reign has produced an entire race of bootlicking slaves, who will 'die for Darkseid' if requested. In 'Secret Six', for example, Scandal Savage employs a nameless Lowlie as a servant. Who gleefully begs for all kinds of abuse, as if torture is as necessary as food for its well-being.
In the Jackie Chan Adventures fanfiction Queen Of All Oni, Ozekai has fanatic Undying Loyalty to Jade (to the point even she is a little unnerved), that apparently is meant to mirror the relationship between Megatron and Inferno (though he seems to have a bit of Lugnut as well).
Probably the most famous portrayal of the character is Dwight Frye in the 1931 film. Indeed, Frye's Renfield is more in keeping with the trope than the character's depiction in the original novel.
In Let The Right One In, Eli's 'guardian' seems to love her, despite her usually treating him callously. Depending on your interpretation, young Oskar may have taken over his role by the end of the movie.
In the book on which the film was based, his motivation was a paedophile's lust for and fascination with an unchanging child; the author of the book, in the course of writing the screenplay, dropped this sub-plot as being one too many and far too squicky.
Eric the Hunchback serves as Renfield for Dracula in Santo y Blue Demon contra Drácula y el Hombre Lobo. Though he plays the usual role of a loyal servant who can move around and do things that a vampire can't, there's an interesting twist to this: It turns out that, even though Eric is mortal, he has become so evil that he is not immune to the film's Achilles Heel for vampires, the Dagger of Boidros: it can destroy him as well.
In the 1996 101 Dalmatians film, Cruella De Vil had a fair number of people loyal to her, but her PA Frederick took the cake with the page quote.
Renfield in Dracula, who could actually be something of a subversion in that, while he certainly seems willing to become Dracula's slave, being locked in at Dr. Seward's sanatorium rather limits his options and the Count seems to more or less ignore him throughout. (Until he finally visits him in his cell and kills him.)
The original is a clear-cut subversion, and his death was also a Crowning Moment of Awesome. Renfield at one point demands that he be moved so that Dracula will not compel him to let him into the house to attack Mina. When this fails, the second time Dracula enters, he grabs Dracula and tries to kill him with his bare hands, while the Count is in mist form. And he would have succeeded, too, if Dracula hadn't used his Hypnotic Eyes.
In The Dresden Files, Black Court vampires can exert mental domination on humans to create permanent mindslaves. They are actually referred to as "Renfields" because Bram Stoker "wrote the book" on slaying Black Court vampires. These Renfields are more competent than most examples of the trope, and more tragic.
Or at least, they are competent within narrow fields: they make great cannon-fodder Mooks, and they might be useful for similarly mindless tasks, but they aren't so good at complex thought, given that their minds have been forcibly ripped away and replaced by unthinking obedience.
The White Court’s thralls (emotionally drained human husks) might also count as this. Red Court vampires can do something similar because their saliva is an addictive narcotic.
This is a semi-official rank in vampire society in Nancy A. Collins's Sonja Blue Trilogy. Humans with some telepathic ability and a psychological disposition to submission are often enslaved by master vampires (via Mind Rape, which an ideal candidate for the job will actually enjoy) and used as personal assistants. The position is referred to as "renfield" (in lower case), but the master of such a servant dehumanizes him/her by addressing him/her only as "Renfield" (upper case).
Similarly, vampires in the Anita Blake series call the humans who serve them, those who have been bitten a few times and are thus somewhat in thrall to the vampire, Renfields. When asked "What did you call them before Stoker's book came out?", the answer was simply "slaves."
In Stephen King's vampire novel 'Salem's Lot, Mr. Straker serves in this role, but subverts it in that he is quite capable as the vampire's daytime operative.
The vampires in Patricia Briggs's Mercy Thompson novels have "sheep" - people who are kept on hand as walking meals - whose Sycophantic Servant-ness varies depending on how the vampire treats them. They all become increasingly subservient to and dependent on the vampire after repeated feedings. If there are enough sheep to keep the feedings infrequent, then the people can stay healthy indefinitely, and there are some benefits (such as cancers being kept in remission), which logically explains why some of them are quite happy with their lot. Stefan, the most sympathetic vampire, does this on purpose, seeking out potential sheep who need a safe haven or medical help. Eventually, the vampire may decide to "turn" a sheep, but this isn't always possible.
Krishna of Someone Comes to Town, Someone Leaves Town is referred to as a Renfield in-story. Unusually for this trope, he begins with self-righteous intentions as a wannabe monster-hunter, but both his sadism and his toadyism are readily apparent early on, and he readily sides with an undead fiend who's blatantly the most evil character in the book.
Damane in The Wheel of Time are forced to become this after some hard core Mind Rape from the Seanchan. There's a scene in the first book where they appear when Nynaeve frees one out of pity, and the damane without missing a beat starts screaming, begging her slaver to put back on her leash. It's a little creepy.
In Charlie Huston's Already Dead and its sequels, the vampires of New York classify humans who know about them based on characters from Stoker's novel. Renfields are willing servants, Van Helsings are enemies, Lucys are wannabees and Minas refrain from judging vampires solely by their nature.
In the Sword of Truth universe, this is essentially what happens to anyone who gets confessed — they are made completely, unconditionally loyal to their Confessor, to the point where they no longer have any sense of self.
The Sycophantic Servant in Young Dracula is actually named Renfield. His grandfather is brought back from the dead in one episode, very angry about not being transformed into a vampire as was promised, meaning that the Draculas have managed to keep multiple generations serving them with this promise they have no intention of fulfilling.
The Weasel-like Tim Stamper from House of Cards is Francis Urquhart's Sycophantic Servant. Subverted in that in the sequel To Play the King he attempts to double-cross his boss after feeling his efforts aren't appreciated. Sadly for Tim, Francis is far and away the more Magnificent Bastard of the two.
Colonel Klink of Hogan's Heroes does this to pretty much every officer that walks in the door, General Burkhalter especially. To a man, they find it annoying.
In the fifth season, Meg of Supernatural is presented as one to Lucifer. Of course, he's planning to kill her, along with every other demon in existence, once he's done with humanity.
The Vorta of Deep Space Nine were genetically altered to regard the Founders of the Dominion as living gods. They are well aware of this, and take it in stride. "What's the point of being a god if there's no one to worship you?"
Knox in Angel is a follower of the ancient demon Illyria before he resurrects her and even more of a slavish follower after her resurrection. Having been dead to that point Illyria had no direct contact with him before her return.
Chip to Cassandra in the Doctor Who episode, "New Earth," to the point where he willingly lets her possess his body, even though doing so facilitates his death.
Pat Patterson and Gerald Briscoe to 'Mr. MacMahon'. What makes them different to most of the other people working closely with Vince was Pat and Gerald legitimately thought Vince was the light of their lives and never aligned themselves with him out of greed, power or fame. Even Vince's own kids back-stabbed him on occasions, but not the stooges.
Wheeler from Pokemon Ranger: Shadows of Almia is the short and stumpy Yes Man to Altru Inc.'s president, Blake Hall. While the others in his circle often utilize high-power Pokemon, Wheeler always attacks with...Bidoof.
Kael'thas has become this to Kil'Jaeden in World of Warcraft due to a mix of Fel Magic taint and his own hunger for power. His blind, zealous and borderline loving devotion to the Legion Lord is very creepy. Though it might have something to do with the fact that he was brought back from near-death by Kil'jaeden's demons. Before that he seemed quite sane.
Ishida Mitsunari from the third game of Sengoku Basara is implied to have been one of these to Toyotomi Hideyoshi in the prequel manga. Unfortunately for anyone who shows the slightest sign of wanting to oppose Hideyoshi, Mitsunari is terrifyinglygood at his job.
In The Elder Scrolls V Skyrim, Cicero of the Dark Brotherhood is slavishly devoted to the Night Mother, the corpse of an important figure in the brotherhood's Religion of Evil that as Keeper is his duty to maintain and protect. He constantly begs to hear her voice call to him, something that only happens to the Listener (aka the Dragonborn). Also, if you decide to spare him later on, he returns in the end of the questline and becomes this to you.
In The Simpsons, Smithers fulfills this role. In the Dracula Halloween episode, he was even dressed literally as Renfield (played by Tom Waits!) in Francis Ford Coppola's Dracula movie.
The Batman: Harley Quinn's obsession with The Joker is so bad, that in an episode wherein he was perfectly willing to leave her in the city while he nuked it, she was about to quit. In one line he gets her back. This is completely in character.
I WILL NOT HIDE! I WILL STAND PROUDLY AND SHOUT MEGATRON'S NAME TO THE HEAVENS!
Played straight (mostly) with Sunstorm, the sycophantic clone of Starscream.
In G1 Transformers, Cyclonus is Galvatron's Sycophantic Servant. His exact level of competence is plot-reliant, but generally he's described as quite powerful and skilled—he could even lead the Decepticons if he weren't so devoted to Galvatron.
Ditto for Shockwave's relationship to Megatron, before The Movie.
In another Transformers example, there's Inferno from Beast Wars, who could probably give Lugnut a run for his money in the obsession department. Perhaps a semi-subversion, as his competence level varies throughout the series (due to being an Ax Crazypyromaniac who thinks he's an actual fire ant), but his loyalty never does (as he's also convinced that Megatron is the Queen of the colony).
Inferno really doesn't have a choice in the matter though due to faulty programming.
On Jimmy Two-Shoes, all of Lucius' Minotaurs are like this, to the point where they willingly sacrifice themselves to use as bait so Lucius can fish.