Dante remains loyal to Miguel even after the latter refers to him as a dumb dog. It turns out Dante was trying to help Miguel stay away from the man who murdered his great-great-grandfather.
Pepita, Imelda's large alebrije cat, has remained loyal to the Rivera family since their deaths. She is ultimately the one who punishes Ernesto De La Cruz for the crimes he's committed against them since murdering her master's husband.
Olaf. Even when he discovers that fire will melt him, he still stays to warm Anna as her curse freezes her.
Anna refuses to give up on Elsa no matter what, to the point of running off into the mountains to bring her back after she freezes Arendelle, even without having any idea whether or not Elsa did it on purpose, as most people believe.
In the How to Train Your Dragon movies, this is the bond which develops between dragons and their riders, and most particularly between Hiccup and Toothless. In the second film, we see that this loyalty actually allows Hiccup to break the Mind Control being held over Toothless by the Big Bad.
Woody to Andy. He won't, under any circumstances, ever turn his back on his owner and religiously believes it's his duty to be there when Andy needs him.
Slinky to Woody, to the point where it almost destroyed him.
Bullseye to Woody, fitting the television series they are toys of. In the beginning of Toy Story 3, he's the only toy who wants to return to Andy's with Woody, only to be stopped because Woody doesn't want him to be alone in the attic.
The Bensons to Gabby Gabby, serving their boss without question even if they know that they would eventually part ways with Gabby.
In the Back to the Future series, Marty McFly shows this towards anyone he cares about. No matter how much his parents' actions in both the past and present annoy or upset him, he will never let anyone get away with hurting George or Lorraine in the long run. He shows this towards Doc as well. In the first movie, he tried everything he could think of to make sure that Doc found out about his death in the future, and in the third movie, he refused to go straight home to the future once he found out that Doc was killed in 1885, insisting on going back to rescue him despite Doc's explicit directions to the contrary and saving Doc repeatedly from Buford once he got there.
Bob from Batman (1989) proves this trope isn't just for good characters. He always, always has The Joker's back covered and follows his boss's every order without question, even when Joker asks to borrow Bob's gun so he can shoot Bob dead just for the hell of it.
Villainous example in Big Game — Hazar's helicopter pilot refuses to leave his boss even after Morris shoots his fellow mook and threatens to do the same with him.
The General's Daughter: Colonel Fowler was tasked by General Campbell with dealing with Captain Campbell after her explosive confrontation with her father. When he found her dead body, he immediately assumed that the General snapped and killed her in a rage for her humiliating improprieties that damaged his command (as it turns out, Fowler was wrong; the General did not kill her). Instead of reporting what he had seen, he covered up for his superior and later, after Brenner reveals his findings, was willing to fall on his own sword to protect his reputation.
Anguirus is Godzilla's most loyal ally, even willing to risk his own life if it means helping his fellow kaiju. Ironically enough, Anguirus was the very first monster Godzilla ever fought. Defeat Means Friendship, indeed!
A prime example of Anguirus' loyalty towards Godzilla can be seen in the 1974 film Godzilla vs. Mechagodzilla. In it, after losing to the Mechagodzilla (disguised as the real Godzilla) he goes to warn Godzilla about the impending danger even after Mechagodzilla had broken Anguirus' jaw and pretty much beat the living daylights out of him.
In Godzilla: Final Wars, Anguirus is released by the Xilians alongside Rodan and King Caesar (Godzilla's other two allies in the Showa series) to stop Godzilla at Mount Fuji. Anguirus and Godzilla have a short conversation before Godzilla gives a sharp backward jerk of his head after Anguirus roars something and charges into battle, as if he's shocked that Anguirus would actually fight against him. Fortunately, Godzilla leaves his three former allies in a comical heap, sparing them from death.
While it doesn't involve Anguirus, the MonsterVerse has a very similar relationship between Mothra and Godzilla. When Ghidora usurps Godzilla's throne, thanks to the foolish intervention of humans again, Mothra remains loyal to him while Rodan and the others follow the three-headed dragon. Hence her title Queen of the Monsters.
Despite the I. rex standing right behind them, all it takes is some close contact with Parental Substitute Owen for Delta, Echo, and Blue to turn against the I. rex and fight to the death to protect him. Charlie also hesitates and appears to reconsider their betrayal as soon as she sees Owen in the firefight. It gets her killed.
Lowery Cruthers, the main member of Mission Control, has this towards both Claire and the titular park. Despite his less-than-pleased view on Claire's more questionable decisions (approving of the Indominus rex, for example) he's there for her until the very end, willingly staying behind even as the rest of the control room is evacuated because, as he puts it, "someone has to". It's a good thing he did, too, because Claire's next plan involved releasing the T. rex to fight the Indominus, something she never could have done if Lowery had left the control room.
One of Eggsy's strongest and most important traits in Kingsman: The Secret Service. His refusal to rat out his friends to the police nearly got him imprisoned and it's clear that even the threat of death won't make him betray the Kingsmen, particularly Harry. In this regard, his Evil Counterpart would be Gazelle.
Diaval maintains this for his oddly Evil-But-Benevolent Boss, Maleficent, after she saves his life. Even if he doesn't always like what she asks of him, he still does it, and when she tries to send him away rather than insist he risk his life fighting by her side, he (with comedic grumpiness) refuses. To her credit, Maleficent genuinely seems to appreciate his loyalty; he's pretty much the only one she trusts at all.
Marvel Cinematic Universe: Arguably the biggest overarching theme of the three Captain America films is the relationship between childhood best friends Steve Rogers and Bucky Barnes. The level of devotion they have towards each other gives them both a reason to live, but also an incredible amount of pain.
The sequel, Captain America: The Winter Soldier, highlights the strength of this relationship, where after it is revealed that Bucky is the Winter Soldier, Steve admits that "Even when I had nothing — I had Bucky." When Sam tries to tell Steve that no matter who Bucky used to be, Steve may have to stop him, not save him, Steve says that he recognizes that, but that he also recognizes that if it ever came to that point, he wouldn't be able to do it. In turn, the Winter Soldier's 70 years worth of programming and brainwashing started to crack from hearing Steve say his nickname once.
The last film of the trilogy, Captain America: Civil War, establishes that there is nothing in the world Steve wouldn't do for Bucky. He becomes a fugitive, desecrating his reputation and legacy, and splits up the Avengers, all just to protect Bucky, who, after the death of Peggy Carter, is the last tie to his old life and the only family he has left. On Bucky's end, according to his actor, the only reason he hasn't put a bullet through his head after regaining his memories and committing suicide is because of Steve. He's so wracked up with guilt over the crimes he committed as the Winter Soldier that he believes that he isn't worthy of the lengths Steve is willing to go to protect him, and deliberately stays away because of that. That being said, killing himself would only cause Steve even more pain and make things worse. The entire conflict of the film boils down to the fact that Steve will never abandon Bucky, for as long as they live.
Rise of the Planet of the Apes: Caesar seems to inspire this in most of the apes he meets. He let the gorilla Buck out of his cage and Buck ended up sacrificing himself to protect him. He defeated Rocket and offered him a cookie and Rocket in time became one of Caesar's most loyal followers.
By the end of Star Trek Into Darkness, it's become obvious that the crew of the Enterprise have developed this with one another after they refused Spock's orders to abandon ship. Even early into the movie, both Scott and Spock continue to refer to Kirk as Captain despite him (temporarily) losing his rank.
Yukio to Mariko, her fictive and later officially adoptive big sister.
Hank McCoy to Charles Xavier. From the Cuban Missile Crisis in 1962 to the rise of the Sentinels in the Bad Future, Hank always remains at Xavier's and the X-Men's side. And he won't hesitate to protect him, either.z In X-Men: Days of Future Past, there are plenty of other things that Hank could do with his time, yet he chooses to take care of Charles during the latter's decade-long Heroic BSoD. Hank may even be a little too loyal because he serves as an enabler by providing a serum which worsens his friend's already bad case of substance abuse.