Berry from Foster's Home for Imaginary Friends totally fits this trope, acting sweet and kind, but being homicidally jealous of anyone who she perceives as a threat to her relationship with Bloo, which is even more imaginary than she is.
The eponymous character of Pucca. She's smitten over Garu, but since he thinks Girls Have Cooties, he's not having any of it. Ever.
Panini on Chowder is obsessed with the title character, to the point where it's scary at times.
"I just wanted to pass laws to make you my property!"
Wendy, Stan's love interest/girlfriend in early seasons of South Park. She went so far as to have a substitute teacher (later revealed to be lesbian) arrested on false charges and locked inside a rocket launched directly at the sun, for the crime of having been the object of a brief crush from Stan.
Azula from Avatar: The Last Airbender fits this when you think about. Her entire Villainous Breakdown is caused by Mai and Ty Lee turning on her for Zuko, who she is thereafter ready to kill on sight.
Sadira in Disney's Aladdin animated series is a "street rat" like Aladdin was, and falls for Aladdin the way he did for Jasmine, but, since he's in love with Jasmine he refuses her. Then she stumbles onto an ancient library of forgotten magic, and the end result is increasingly dangerous plots to get rid of Jasmine and take Aladdin for herself, including rewriting reality at one point and altering his mind. Eventually she gets over her crush and befriends the gang, but almost has a brief relapse when she believes they don't trust her.
Quagmire marries one (Joan) in the episode "I Take Thee Quagmire" in Family Guy. Peter convinces him to get a divorce using champagne and Lois's breasts, and when he tries to broach the subject she threatens to cut herself with a kitchen knife and then cut him.
"And I love you too, Crazy Woman I'm Having Second Thoughts About!"
Meg is Yandere for Brian in "Barely Legal".
Meg, in general.
Stewie has also shown to be quite the Yandere towards whoever he may express an attraction to or fall in love with, male or female.
During one episode of Teen Titans, Starfire shows a few Yandere tendencies when Kitten blackmails Robin into going to her high school prom with her. At one point, she crushes the front end of a car with a single punch. She stops once the forced date plotline is resolved.
In one Looney Tunes cartoon, Daffy must exorcise a possessed heiress, though there's open room for interpretation that she was actually one of these.
And there's "Just The Two Of Us", where Trixie goes batshit insane on Timmy.
One episode of The Oblongs had Helga Phugly become one towards Milo because she wants him to take care of her due to her parents being absent.
The Planet Express Ship from Futurama. She tries to kill herself and the rest of the crew after Bender breaks up with her.
Not to mention Bender himself towards Fry, albeit platonically. When Fry discovers his old dog Seymour's corpse fossilized in a museum, Bender becomes increasingly jealous until, when Fry announces he's going to clone Seymour, Bender grabs the fossil and throws it into a live volcano (in a near-literal Kick the Dog), screaming that he's never, ever going to let himself be replaced as Fry's best friend if he can help it.
The difference here is that he only wants to have sex with her, against her will. He doesn't actually love her.
On American Dad, Francine Smith has shown these tendencies towards her entire family, but most of the time it is directed toward her son Steve. The whole thing gets taken to an especially creepy length in the episode Iced Iced Babies.
Francine: You can stay right here. With mommy. Forever.
The Simpsons: One-shot character Julia was one of these towards Homer in the episode "Homer Of Seville".
And the Pierce Brosnan Ultrahouse on "Treehouse of Horror XII."
Cookie BaBoom, Mayor Tilton's ex-girlfriend on The Mask episode "Flight as a Feather." Uses a variant of the Stock Phrase, "If I can't have you, then no one can" (on the episode, it was, "If I can't have Mortimer, then no one can!" Standard. Is still bitter over being dumped by Mayor Tilton? Okay. Uses two strategically placed explosive belts strapped around her body to kill herself, the mayor, and everyone else around her because of the deep-seated resentment she has over being dumped. Bingo!
Also Nergal Jr as, like Nergal, his dad, wants desperately to make friends but if people refuse to be his friends he freezes them and takes their form.
Heloise on Jimmy Two-Shoes. Among the things she's done when Jimmy hasn't noticed her is doing a Freaky Friday Flip on Beezy and Cerbee and fusing Jimmy and Beezy together. There's also "Best Bud Battle", where Heloise chains Jimmy up to keep him from leaving her and locks him in a tiny cage.
Whitney Stane from Iron Man: Armored Adventures. Her feelings for Tony appear to be yandere, becoming violent if there is a possible threat to Tony's life, even going so far as to try to kill her own father as she believed he was trying to harm Tony.
Breach towards the titular Generator Rex...sort of. While she does have an unhealthy obsession with him (kidnapping him to make him her boytoy, and refusing to let him go even when her life was in danger because of him) it's never been confirmed that she likes him romantically. (If we take her literally, she just thought Rex would be a great action figure.) There's enough evidence for fans to run with, though.
For a definite example, there's that Creepy Child minion of Breach's. She's never named, but she's absolutely obsessed with making Breach happy and tries to kill Rex when she realizes Breach likes Rex more.
Yo towards Chum Chum in Fanboy and Chum Chum. She's obsessed with taking him home, dressing him up in a sailor outfit, and putting him on her toy shelf. She even trapped him in a giant virtual pet case at one point. Also a Genki Girl.
The Bride of Gingy in Scared Shrekless. . . .
Mother Gothel from Tangled is this to Rapunzel, though it's not for Rapunzel herself but for her hair which keeps Gothel young and beautiful. Towards the end, she gags Rapunzel, puts chains on her and drags her to the basement so she won't escape.
In an episode of Disney's Hercules cartoon, Hercules gets an idea when he learns about Pygmalion and tries to make his own dream girl out of clay. Unfortunately, being a hormonal teenager, all the personality he can think to give her is "in love with me". Aphrodite, who Hercules has called to assist in this endeavor, snarks on the poorly-thought-out idea, but brings her to life anyway. Naturally, after Hercules gets tired of her Clingy Jealous Girl tendencies, she shows off her Yandere tendencies. By the end of the episode, Aphrodite gives her a real personality... one that just doesn't care much for Hercules at all.
Somewhat Elena Validus turned into this to Ben in Ben 10: Ultimate Alien episode "Revenge of the Swarm".
While not murderous (probably), Sierra from Total Drama World Tour shows many signs of this towards her crush Cody.
The Problem Solverz: Katrina Rad is obsessed with Roba and stalks him in "Magic Clock". She then steals a time-freezing clock so the two of them can be together for eternity and tries to marry him against his will. When Roba refuses to kiss her at their wedding, she threatens to rip a hole in the fabric of the space-time continuum.
In one episode of The Boondocks Granddad meets an incredibly hot woman over the internet. We later learn that she's also a martial arts master literally Raised by Wolves, has a very bad history with previous boyfriends (whom she killed) and has a friend who gives her terrible advice. The episode ends with the Freemans managing to calm her down after which, her friend drives her to kill herself, to the relief of the other characters.
Amora the Enchantress in Avengers Earths Mightiest Heroes is this to Thor. She takes out her anger from being rejected by him in battle with Thor and his allies frequently. This follows along with her comic book character.
On Squidbillies Early Cuyler for his ex-wife Krystal
"I love you (waves knife around), Imma kill you bitch."
An episode of Regular Show has Starla, Muscle Man's old girlfriend. She falls in love with Mordecai, and he starts dating her just so he can get her and Muscle Man back together. But when Mordecai can't take it anymore and breaks up with her... well...
