It Must Be Mine

""I said to the Red Bull, 'I must have them! I must have all of them, all there are!""

- King Haggard, The Last Unicorn

There is a valuable thing out there. It's one-of-a-kind, which makes it probably the greatest prize ever.

That means someone wants it, just for that reason, by any means.

Note it's not the artifact hypnotically seducing the person into wanting it. The person is just that selfish or greedy. Nor is it something earned. It's just outright taken. It also doesn't count if someone steals it for the thrill of the theft, or to just show how good a thief he/she is. Either the thief has to actually want it, or he/she is stealing it for someone who does.

Note that this rarely ends well, no matter whether the story is idealistic or cynical. That person stole something they shouldn't keep. In idealistic stories, it's too important to lock away. In cynical stories, it's too dangerous to try to lock away.

Anime and Manga

 * That guy in Pokémon 2000 who captured the Legendary Birds.
 * His name is either Gelarden or Lawrence III, depending on the language.
 * Lupin III might be a master thief, but he and his posse actually want a lot of the valuable things they steal.
 * The actions of Fiamma of the Right, the Man Behind the Man of the Roman Catholic and Russian Orthodox Churches in To Aru Majutsu no Index, are guided solely by his obsession with possessing Touma's Imagine Breaker and becoming more powerful than God himself.

Comic Books

 * There's also a Marvel Comics character named The Collector, who collected unique specimens from across the universe.
 * The original motivation of the Batman villain the Cavalier was to obtain unique pieces that he was unable to purchase for his collection.
 * Larfleeze wants everything, but especially rare things, like Lantern rings he's never seen before.

Film

 * Most of the characters in The Maltese Falcon will go to any lengths to obtain the fabled bird.

Literature
"Haggard: They are MINE! They belong to ME! The Red Bull gathered them one-by-one and I bade him drive each one into the sea! I like to watch them. They fill me with joy. The first time I felt it, I thought I was going to die. I said to the Red Bull, 'I must have them! I must have all of them, all there are! For nothing makes me happy but their shining, and their grace.' So, the Red Bull caught them. Each time I see the unicorns -- MY unicorns -- it is like that morning in the woods, and I feel young, in spite of myself!"
 * There is a story where a princess (or maybe duchess) hears someone in her land has a talking cat. She wants it for herself, so she throws the owner in her dungeon. One scene has her riding in her carriage, carrying the cat in her hand muff, and the cat escapes by biting her hand.
 * All of the unicorns, in The Last Unicorn, collected by King Haggard.


 * In Robert Asprin's fantasy novel Myth Directions, Tanda the Trollop assassin ropes protagonist Skeeve into her attempt to find a truly one-of-a-kind item to give to Aahz (a scaly green Pervect) as a birthday present. She ultimately settles on a ghastly frog-scuplture trophy that has no value except as the prize for a heated sports competition between rival cities. Their robbery attempt goes horribly wrong, and things only spiral further out of control from there.
 * To add to it, Skeeve tries to keep Aahz from finding out about any of this because he knows the lecture he'll get for having done something like this. When Aahz does find out and asks incredulously "This ugly statue is what's behind all this?", a frustrated Skeeve responds "Yeah, because we wanted to give it to you. Happy birthday." After a stunned minute, Aahz decides It Must Be Mine and the group alters their escape plan to include the theft of the trophy.
 * In the first Acorna book, Hafiz Harakamian sought to add Acorna to his collection of rarities as the crown jewel. A desire with quickly disappeared once he learned she wasn't some sort of mutant, but a lost member of an space-faring race.
 * Simon R. Green's Nightside books feature the Collector, who is reclusive, petulant and somewhat childish in his singleminded devotion to things that - in his opinion - only he can truly appreciate.

Live Action TV

 * The Star Trek: The Next Generation episode "The Most Toys". The thing that was stolen was Lieutenant Commander Data. It probably wasn't one of Kivas Fajo's better business plans...
 * An episode of Jonathan Creek had a woman and her daughter steal a valuable statue to spite its owner (the mother's brother). When the daughter went to recover it she was murdered by an Ax Crazy, sociopathic pimp who decided he wanted it simply because he heard it was valuable.
 * In Lois and Clark, a baddie captures Superman, and uses Lois Lane as hostage to make sure he doesn't escape. Which he does, via New Powers as the Plot Demands.
 * The Avengers episode "The Man from Auntie" features a villainous firm which collects items like this for... collectors. The "item" they've currently stolen and plan to auction off is Emma Peel.
 * Doctor Who:
 * In The Seeds of Doom, two alien pods are found in the Antarctic permafrost. The psychotic millionaire Harrison Chase sends a team of thugs to get them, because they're unique. The pods cause vegetation to kill animal life, endangering the world.
 * In City of Death, a key plot-point involves the theft of the Mona Lisa to be sold (along with six identical paintings) to finance Count Scarlioni's temporal experiments. Inspector Duggan observed to the Doctor at one point, "There are at least seven people in my address book who'd pay millions for that picture, for their own private collections. It would be an expensive gloat, but they'd do it."
 * At least one Mission Impossible episode had a millionaire who had a national treasure that the original nation wanted back, and it was important they get it for reasons of international politics. The current owner was not cooperative.
 * Comes up in Dads Army of all places where Mainwaring is trying to buy some oranges at a charity auction from the warden, who predictably is doing everything in his power to stop him getting them. Eventually Wilson tells Pike to buy an orange for Mainwaring, but neglects to tell the captain, resulting in Mainwaring entering a furious bidding war against himself. He eventually ends up paying ten shillings for it (when the first one sold for a couple of pence).
 * Kamen Rider Diend of Kamen Rider Decade has a 'treasure' in each episode that he is seeking. He alone knows what makes something a 'treasure,' and he only wants it because it is one, and rarely for what it actually does (some of 'em are important to the plot.)

Video Games

 * Oliver, the Duke of Tanas from Fire Emblem 9 and 10, toward the royal heron laguz.
 * Chandra in Eternal Darkness says this about one of the artifacts of the Ancients, before she knew what exactly it was.

Web Comics

 * Dork Tower: This is actually Igor's Catch Phrase, though it seems like it doesn't so much have to be a unique thing as just press his nerd buttons.

Western Animation

 * There was a Thundercats episode where some queen tried to cage a magical songbird (although its song didn't come across as beautiful as we were told).
 * Superman: The Animated Series, episode "The Main Man": An alien called the Preserver has a private zoo in which every creature is the last of its kind; in the episode, it attempts to add Superman, the last Kryptonian, to its collection by hiring Lobo to capture him.
 * Once Lobo succeeds, the Preserver captures him as well, since he's the last Czarnian. Lobo and Supes team up. Hilarity Ensues.
 * In a Treehouse of Horror episode, Comic Book Guy is the supervillain "The Collector" who collects Lucy Lawless, the Fourth Doctor, Matt Groening, etc. He also owns the only working phaser ever made.