The 13th Warrior gives us Ahmad ibn Fadlān ibn al-Abbās ibn Rāšid ibn Hammād. At first, his Viking companions assume his name is merely "Ibn".
Batman: The Movie has Catwoman using the alias, Kitayna Ireyna Tatanya Kerenska Alisoff. Fortunately, she also goes by the acronym Kitka, or more commonly Miss Kitka.
Dr. Ngegitigegitibaba from Bedazzled (2000). Can double as The Unpronounceable, as the film's sole reason for him having that name is for Elliot to show his lingual proficiency by pronouncing it correctly.
The Cat from Outer Space usually goes by "Jake", because his human friends cannot remember "Zunar J5/9 Doric47".
In A Christmas Story, the one thing Ralphie wants for Christmas is an Official Red Rider, Carbon-Action, 200-Shot, Range-Model Air Rifle.
In Compañeros, Vasco's name isn't so much long as difficult to pronounce: it's Modesto Servando Iruretagoyena.
In Day of the Evil Gun, Bandito leader Jose Luis Gomez de la Tierra y Cordoba DeLeon introduces himself by his full name.
El Dorado: His name is Alan Bourdillion Traherne ("Lo-ord a'mighty!..."). That's why most folks call him Mississippi.
Leeloo, in The Fifth Element: Leeloo Minai Lekarariba-Laminai-Tchai Ekbat De Sebat; thankfully, she goes by "Leeloo Dallas" a few scenes later. The Divine Language website (dedicated to the film's made-up Classical Tongue) translates the entire thing to "Precious Gem of the Earth and Honourable Defender of Light and Life."
Otto: How do you do, Mrs. Leach? I'm, uh, Harvey. Manfred... jen... sen... den. I'm with the CIA.
One protagonist of The Good, the Bad and the Ugly, Tuco Ramirez. His full name is Tuco Benedicto Pacífico Juan Maria Ramirez, "known as the Rat".
In Greaser's Palace, Jessy has been sent to warn the townsfolk of the evil devil-monster named Bingo Gas Station Motel Cheeseburger With A Side Of Aircraft Noise And You'll Be Gary Indiana.
President Dwayne Elizondo Mountain Dew Herbert Camacho in Idiocracy.
From The Imaginarium of Doctor Parnassus there's Percival St. Antoine della Touraine et Sansepolcro da Piemonte the Third. Everyone just calls him Percy.
"Your honor, I call Heinz, the Baron Krauss von Espy" "Call Heinz, the Baron Krauss von Espy!" "Heinz, the Baron Krauss von Espy!" etc...
Also, Marilyn herself, who adds the surnames of her ex-husbands to her own name. Wheezy Jones asks "is that one person?"
It's not a character name, but in Iron Man, people always respond to the Strategic Homeland Intervention, Enforcement, and Logistics Division with "Wow, you should shorten that." They do by the end of the film, to S.H.I.E.L.D..
Phil Coulson: I'm agent Phil Coulson, with the Strategic Homeland Intervention, Enforcement and Logistics Division. Pepper Potts: That's quite a mouthful. Phil Coulson: I know, we're working on it.
The full name of the noble protagonist in Kate & Leopold is revealed to be Leopold Alexis Elijah Walker Gareth Thomas Mountbatten. The cop he is informing of this is not amused.
Somewhat subverted in the remake of Mr. Deeds. Winona Ryder's character (lying) says she is from Winchestertonfieldville, Iowa. Later in the film, Adam Sandler's titular character somehow finds (and takes her to) Winchestertonfieldville, Iowa.
The classic Laurel and Hardy short "The Music Box" features Professor Theodore von Schwarzenhoffen, M.D., A.D, D.D.S.,F.L.D, F.F.F unt F. Thus combining this trope with Overly Long Title.
In The Pest, the eponymous Pest states his full name is Pestario Rivera Garcia Picante Salsa Vargas.
Most of the film titles of the Sartana series, the longest one being Una nuvola di polvere... un grido di morte... arriva Sartana (translated as Light the Fuse... Sartana Is Coming).
Spy Kids: Carmen's full name is Carmen Elizabeth Juanita Echo Sky Brava Cortez. Her brother's full name? Juni Rocket Racer Rebel Cortez. Rocket, Racer, and Rebel are the names of three of director Robert Rodriguez's sons.
In Texas Across The River, Alain Delon's character introduces himself as Don Andrea Baldazar, el Duque de la Casala. Dean Martin's character replies, "And what do folks in a hurry call you?" before deciding to dub him "Baldy". In the real world he'd probably be known mostly as Casala.
Titanic (1997): When Rose introduces herself to Jack, he acts like he wouldn't be able to remember her whole name as a way to show how simplistic of a person Jack is next to the fancy Rose.
Jack: I'm Jack Dawson. Rose: Rose DeWitt Bukater. Jack: I'm going to have to get you to write that one down.
, but that's just his nickname. The actor is credited on IMDB with the entire full name of this character.