Ghost in the Shell: Stand Alone Complex has an interesting example where an ex of Pazunote The same person that said he never sleeps with the same woman twice. dons a perfect prosthetic duplicate body of his and frames him for two murders. It goes further in that she claimed to still love him, but believed that by being inside an exact copy of his body she had his "ghost" inside of her. Then she tries to kill the real Pazu. It's not quite made obviously apparent that the real Pazu won the fight.
While not in total control of her faculties, Queen Mirage of Happiness Charge Pretty Cure is certainly this. Turns out she was a Shrine Maiden who fell in love with Blue 300 years ago. While he returned the feelings, he felt that, as a God, he shouldn't be attached to any one person and rejected her. She didn't take it too well, especially since it seems that Dark Mirror took control of her because of it.
Ray Palmer and his ex-wife, Jean Loring, in Identity Crisis. She gets committed to Arkham Asylum at the end. (Despite, er, not even living in the same state. Apparently there are no other asylums in the DCU.)
Played straight with eventual fights involving Scott's ex-girlfriends Envy and Knives. For fairness's sake, apparently, Ramona handles these fights.
Tony Stark broke up with Kathleen Dare because she was being too clingy. So, she shot and crippled him.
When Tony is later assumed to have died from an illness she shoots herself so she can join him in the afterlife.
Red She-Hulk a.k.a. Betty Ross Banner. One of Hulk's wives finally doesn't stay dead, and this happens.
Kimura, X-23's Ax-Crazy former handler, strongly comes across as a crazy stalker ex after Laura escapes her and the Facility. She's obsessed with getting Laura back, making several notable attempts to recapture her. When she finally does manage to retake her during Laura's stint on X-Force, she's actually licking her lips as she takes a chainsaw to Laura's arm for "being a bad girl."
Extended Stay gives us Mistress's ex-boyfriend William Queen in Chapter 7. He jumps up on stage during her date with the Warden, loudly accosts her and even threatens her boyfriend at knife-point! It doesn't really help that he was an inmate at the latter's prison.
The obvious example must be Glenn Close's character in Fatal Attraction. The film that gave the world the term bunny-boiler.
My Super Ex-Girlfriend was made on this trope. It would have done a lot better, if they put some actual effort into the plot.
Tanis and Kitiara have this kind of relationship in Dragonlance, and feelings definitely linger on both sides (though in Kitiara's case, those feelings aren't much deeper than "lust and amusement"). From a technical stance, Kit's less "psycho" than "amoral and ambitious", but she still fits the trope.
Given how Kitiara becomes insanely jealous after hearing that Tanis is traveling with his ex-girlfriend Laurana (especially after hearing how incredibly beautiful Laurana is) and how this jealousy causes Kit to become obsessed with Laurana to the point of relentlessly stalking the elfmaid and trying to give her a Fate Worse than Death, Kitiara definitely fits as a "psycho."
The X-Files: Fox Mulder had two of these — the first one was mostly just annoying and appeared as New Old Flame to dump him again, but the second one turned out to be working for the Government Conspiracy and having some kind of weirdly ambiguous relationship with the Smoking Man.
For a few episodes Blair Waldorf on Gossip Girl becomes this to Chuck. Most people thought it was fair enough, since he was giving his new girlfriend incredibly expensive watches, crediting her with making him a better man and treating her like a queen, when he'd whored Blair out to his uncle and then slept with her greatest enemy.
Another Whoniverse example: John Hart, Jack's ex-"partner" who shows up in the second season premiere of Torchwood. He reappears in the finale, holding Grey, Jack's long-lost brother, hostage and tearing Cardiff apart for no apparent reason but to get Jack's attention. It turns out to be a subversion, as Grey is the one in charge and has a bomb fused to John's wrist, forcing him to obey when he'd rather be anywhere else.
Jack's bad luck with men continues in Torchwood: Miracle Day, when his boyfriend Angelo's Catholic gayngst leads him to do terrible, terrible things.
How I Met Your Mother parodies this in "The Platinum Rule": at the end, Barney assumes Wendy the Waitress is this. She is actually taking their break-up really well, but he interprets her total maturity about the matter as a facade hiding a desire to murder him—for at least two years.
Being as the show focuses heavily on dating, a number of straight examples show up, as do a number of women who never quite become girlfriends.
A fun variation comes up when Lily starts following the girl Marshall's interested in, making Lily, by all rights, the crazy ex. At the end, Marshall concludes that Lily is one of the craziest people he's known. It's the episode they get back together.
iCarly: Lewbert's ex-girlfriend Marta Trundel in iFind Lewbert's Lost Love.
Oz. Unit Manager Tim McManus forms a brief liaison with prison guard Claire Howell. When he tries to break off the relationship she assaults him in his office (to the delight of watching inmates), then charges him with sexual harassment when he tries to fire her. Howell's later actions definitely cast her in the role of a villain — she assaults (and in one case murders) inmates, as well as using them for her own sexual gratification whether they want it or not.
24. Two characters, Jack and Tony, dating the same person, Nina.
Davina from Material Girl Tv Series its revealed at the end of the series that most of her attepts to hurt Ali were in fact directed at Marco, Ali just got in the way.
Virginia from Sunset Beach. She even tried to have her ex's current girlfriend impregnated by another guy using a turkey-baster! in a memorable scene.
Havoc and Sakura in Renegade.
The Vampire Diaries has Katherine Pierce, who was a former love interest of both Stefan and Damon Salvatore during their human lives. Katherine enjoys torturing and playing with both Stefan and Damon and has threatened Elena numerous times. Katherine appears to be obsessed with Stefan in particular and seems to be both jealous of Elena and in competition with Elena for Stefan's affections.
Rose from Two and a Half Men both fits and subverts this. Although becoming friends with Charlie, she does manage to drive away his love interests and refuses to give up. Plus, every main character has called her crazy at some point. She's also pretty civil in her stalking, despite it being creepy.
In the ninth season premiere it's revealed she murdered Charlie for cheating on her.
After Dexter finally cut it off with Lila in Dexter, she nearly killed him and his girlfriend's children.
Niles's ex-wife Maris from Frasier probably qualifies, although more subtly: when she begs for him to come back to her, and he rejects her on the grounds that she has treated him like dirt all his life, she goes into a Tranquil Fury and proceeds to utterly screw Niles out of every cent he has with a team of Amoral Attorneys, just for the sake of revenge (she was filthy rich already).
Also Niles' second ex-wife Mel, who after feeling justifiably humiliated when Niles abandoned her to run off with Daphne Moon, exacted her pound of flesh by insisting he go through the motions of marriage with her until such time as she felt they should seperate and divorce. Well, everyone said she was an expy of Maris...
In Zoey 101, we have Chase's short-lived girlfriend, Rebecca. Admittedly, she was psycho while she and Chase were still dating; she demanded Zoey no longer have any contact with Chase, and that Chase stop being friends with Zoey. It was the reason they broke up. Rebecca, of course, took this to mean Zoey was the reason they broke up, and in a later episode ends up blackmailing her into doing all sorts of embarrassing things.
In one episode of Boy Meets World, Shawn becomes the target of three of these who band together to sabotage his latest relationship because they think he is leading her on like he led them on.
Gabe of The Office is a psycho ex boyfriend. He becomes one after Erin breaks up with him. He follows Andy, (whom Erin really likes) to the men's room and threatens him and follows Erin into the ladies room and pleads for her to take him back.
Vera Bates of Downton Abbey to the extreme. She refuses to let Mr. Bates divorce her, blackmails him for everything he owns and then pulls an I Lied and tries to ruin his reputation and those of his girlfriend Anna and the family that employs them anyway, and finally commits suicide in order to frame him for her murder. All this to a man who went to prison in her place for theft beforehand.
On My Name Is Earl, Earl had a girlfriend of three weeks named Jessie, as of the night he had his Accidental Marriage to Joy. Jessie worked as a secretary at the bail bondsman's office Earl frequented. When Joy had an arrest warrant for failing to appear in traffic court 6 years later, Jessie puts herself through Training from Hell so she can get revenge on Joy for a) stealing Earl from her and b) knocking out her two front teeth. It doesn't work, and she gets her veneers knocked out too.
Lauren to Mitchell in Being Human. Justified since Mitchell turned her into a vampire and then dumped her to deal with her new undead status herself.
"Before He Cheats" by Carrie Underwood, a song about vandalizing a guy's car because he is "probably" in the process of trying to score with another woman.
The title character of the Dixie Chicks' "Goodbye Earl," who puts poor Wanda into intensive care after she gets fed up with his abuse and files for divorce. The girls themselves might also qualify given what they do to him in revenge for this. I wouldn't want to be those girls' next boyfriend.
And also to Carl Johnson, although he never actually wanted to be her boyfriend.
Shel Jelavan is this to Zayne Carrick in Knights of the Old Republic. However, she's sort of sympathetic, and they do genuinely patch things up once the collossal misunderstanding is resolved.
Adele from Arc Rise Fantasia has a definite air of this about her. She was only ever a girlfriend in her own mind (and hopes), but evidently that was enough to trigger a violent murderous storm of Yandere.
Don't forget Oasis going after any female seen speaking to Torg. Inanimate objects are not safe from her wrath if she sees them as rivals for Torg's love, either.
Monica has gone far beyond Gwynn; refusing to understand that Riff broke up with her and using every Mind Screw trick to keep him too intimidated and confused to get away. Subverted in that she has a method to her madness, double-subverted in that her ulterior motive is also crazy.
Mona Montrois in C'est la Vie, over Louis and Tiffany. Her behaviour towards tiffany gets more and more unhinged until she ends up sectioned in a psychiatric hospital, and is afterwards released to intensive therapy, nearly driving her shrink insane.
Damara Megido of Homestuck is an arguable example; learning that her (then) boyfriend, Rufioh, was secretly dating Horuss on the side drove her past the Despair Event Horizon, and Meenah's needling over the matter made her snap and go completely mad. In addition to killing Meenah and rendering Rufioh quadriplegic, it's heavily implied that she killed Horuss as well. It's implied that she still has lingering feelings for Rufioh, even though he doesn't really want to date her anymore.
Shannon in Echo Chamber's Trope Of The Week series has an entire episode dedicated to her psycho ex nature. She stalks him, threatens other girls he's seen with (Dana in particular), and seems to believe they are still dating. But the kicker? She forced him to propose to her. Oh, and she regularly cheats on him. Hooray for the Double Standard.
One could take the above spoiler as Tom proposes to her, but then gets cold feet and runs off. He takes Shannon back and now they have sex every so often.
Hell, Terra period. She was much worse in the comics! When she had the Titans (aside from Nightwing) captured, Gar pleaded with her, only to have her not only laugh in his face but sneer that kissing him "made her want to puke." Even better is that she was actively boinking Deathstroke (who was actually afraid of her). Poor Gar still can't fully acknowledge she was a traitor all along.
Comics Terra doesn't really apply to this trope since she was psycho before Changeling met her and any romance between the two of them was all an act on her part. Animated Terra however fits the trope to a T. She was pretty much on the neutral side of things, agreeing to sell the Titans out (well four of the five) as a favor to Slade for helping her control her powers, but not exactly on the evil side until Beast Boy told her she had no friends which turned her over the dark side completely until her Heel-Face Turn three episodes later.
Goliath and Demona of Gargoyles, with emphasis on the "psycho"
This is a case where feelings do still linger on both sides, but both characters recognize that they're now too different to ever be an item again (trying to kill each other almost every time you meet will do that to you). Doesn't stop the shippers.
Notably, her insanity was there when they were dating. He just had a change of heart and couldn't stay with a megalomaniac.
Trixie Tang from The Fairly Oddparents briefly became this to Timmy after he wished they were the only two people on earth.
Blackarachnia from Transformers Animated is actually this to Optimus Prime, after he and Sentinel accidentally leave her behind on a spider-infested planet. Sort of, anyway: she was actually Sentinel's girlfriend, but it's pretty obvious Optimus has a thing for her, which she returns after being mutated; she refuses to trust him, though.