Forgotten Friend, New Foe aka: Forgotten Childhood Friend
"Every murderer is probably somebody's old friend."
—Agatha Christie, The Mysterious Affair At Styles
The shining gem of the Non-Serial Movie, an antagonist who is inevitably related (especially romantically) in some way to one of the characters. The trouble occurs when they revisit, and the current situation (say, a character's new Love Interest) is one that doesn't suit them at all. For maximum effect, The Reveal pulled off near the very end gives a little twist to the "villain" as well as some surprising insight to a relevant character. These work well for such a movie because it is much easier to add backstory without affecting things later, and is often never mentioned again in the television series proper.
Subtrope of Childhood Friends. Compare Beleaguered Childhood Friend. Contrast Forgotten First Meeting.
Forms the basis of one of the Urusei Yatsura movies: Ataru made a Childhood Marriage Promise to a girl he frequently saw on the playground. Fast-forward ten years, and we discover that not only was she an alien princess (with an entire army at her disposal), she's never realized that the proposal was meant as a joke. Understandably, Lum isn't appreciative of this state of affairs; neither is Ataru, but that's because he isn't getting any tail out of it.
Ukyou first appeared in Ranma ½ in this role, but did a Heel-Face Turn (such as it was) and put herself in the running for Ranma.
Sara's long-lost brother Ralph in Soukou No Strain is revealed near the end of the first episode, after the Union has been praising him and Sara herself chasing after him for years, to be the most fearsome villain the series had to offer.
The demonic Duel Monsters card Yubel in Yu-Gi-Oh! GX was revealed to have been owned by Juudai back in the day. He didn't forget her on purpose, though, having his memory suppressed due to the nightmares he suffered when she was in pain up in space.
Very similar situation in Gundam SEED, which has Athrun Zala and Kira Yamato fighting each other despite having been the best of friends earlier in life (something which causes both massive amounts of Angst).
Mendou in White Album has aspects of this, only she is a benevolent, and not a hostile figure (as far as these things can be counted).
In Revolutionary Girl Utena, we find out pretty early on in the series that Utena met Touga and Saionji when she was young. Shortly after that, she met Dios. Shortly after that... she met Anthy.
Justified in that she only saw the boys once before the series and probably wasn't even paying attention because she very depressed and traumatised after her parents's deaths. In the case of Anthy, she was given Laser-Guided Amnesia.
A variation in Pokémon Special. Ruby and Sapphire both remember having a childhood friend, but they don't remember what the other looks like. As such, they don't recognize each other when they reunite five years later and get off the wrong foot. Justified by the fact that they both had their personalities completely flipped since they were kids.
Makarov, the Big Good of Fairy Tail and master of the titular guild, recognizes Grimoire Heart's master Hades as Purehito, his predecessor as the master of Fairy Tail.
Shiroemon in Dorabase, who befriended Kuro during their days in school. They broke off the friendship after Shiro refuses to stay as he is (being good at nothing). From what is known from the flashbacks, he believes that nobody can be counted on to succeed by one's self after what he's been through. He is initially presented as a Jerk Ass, selfish character who openly shows disrespect towards his teammates. However Character Development kicks in soon enough and he gets better, and his relationship with Kuro being more rivalry than dislike, but he is still in the Whiters, Dora's Worthy OpponentArch-Enemy.
90s Superman villain Conduit may have started this as a trend in comic books. An old schoolmate who always felt overshadowed by Clark growing up (with an emotionally abusive father not helping matters), his grudge turns murderous when he figures out the truth and assumes Clark was only better because of his powers.
Tommy Elliot / Hush from Batman - a childhood friend of Bruce Wayne's who turns out to be the villain behind the arc introducing him. His jealousy of Bruce stemmed from him hating his parents and envying the "freedom" Bruce received. And, blaming Bruce's father for saving the life of one of his parents, preventing him from being free.
Another Batman example is Dan Kingdom / "The Enforcer" from the early 70s "Manhunter" arc. Bats finally figures it out (due to an idiosyncratic fighting style) and delivers a pretty savage beatdown while noting that his opponent was completely the Enforcer now and no longer his old friend Dan Kingdom: "The Council may have planted a clone's body to throw us off the trail, but it was still you who died."
Live Action TV
In the season 2 episode of Buffy the Vampire Slayer "Lie To Me", Billy Fordham—"Ford"—an old friend of Buffy's from LA, shows up in Sunnydale. He has figured out she's a Slayer... and that's just for starters. He makes a deal with vampires to bring them Buffy and some VampireWannabes to feed on, so that they will turn him in exchange. He's trying to avoid dying of cancer.
"Conversations With Dead People" had Webs, an old classmate of Buffy's, recently being turned into a vampire. They have a long friendly conversation before fighting to the death.
Doctor Who has Mels, who grew up with Amy and Rory as their best friend, but whom the Doctor (and audience) have never met before. After she explains she skipped appearing at their wedding, she pulls a gun on the Doctor in order to "Let's Kill Hitler!" but gets fatally shot during the escapade. This is where it gets complicated... as Mels begins to regenerate.
We then get The Reveal that Mels is actually Amy and Rory's daughter, Melody (named after herself), who was kidnapped by the Silence and raised to kill the Doctor. After an unforseen event caused her to regenerate from a little girl at the end of "Day of the Moon", she decided to spend her second incarnation growing up alongside her parents. Which brings us to her nearly regenerated third incarnation... River Song, who's future-self both Amy and Rory have met many times beforehand.
Magic: The Gathering has Gerrard, hero of the Weatherlight saga, pitted against Volrath, evil evincar of Rath and Gerrard's jealous adoptive brother from his youth.
More literally, everyone, especially Brown. When Jennifer was found after the massacre, she had amnesia, and only remembered years later.
In Epic Mickey, Oswald the Lucky Rabbit is Mickey Mouse's older half-brother who was lost to Walt Disney over a legal dispute and replaced by Mickey himself. Judging from what information we have on the game now, it's not going to be pretty when he and Mickey first meet.
Persona 2: Innocent Sin: While everyone else in the "Masked Circle" group suffers from Forgotten First Meeting, the last member, Jun, still does remember that they nearly killed Team Mom Maya and built up a grudge... and became the enemy known as the Joker for the first 3/4ths of the game.
Used in Final Fantasy VIII when it's revealed that all of the SEED members in the main party had grown up together in the same orphanage, but most of them had forgotten. A Justified Trope, as amnesia is an established side effect of letting Guardian Forces take up space in your brain.
Soul Nomad & the World Eaters state that Lobo, one of the villains was this to Christophe, with this being a big part of the former's Start of Darkness. This plays a part in The Demon Path, where if Revya states that she trusts him, he bursts into maniacal laughter and states that only fools trust..