This is a relatively rare trope. Because of its nature, it has a near certainty of violence attached to it.
A Predator may refuse to harm his natural Prey or enemy, instead choosing to protect it. The reasons may vary widely: odd friendship, orphan, parental abandonment, injury/ helplessness, etc. The protection may extend to only one individual, to a small group (such as a family), or to the entire species or group. Said protection likely will include protection against the Predator's own kind. This trope is different than Morality Pet in that the decision to become a Protector is irrevocable and is likely to invoke a heel face turn on the part of the Predator. Said protection may be permanent or temporary depending on whether the Predator's intent is to return the Prey back to his family, group, or natural environment. (The intent is always what's best for the Prey, yet another difference between this trope and Morality Pet.) Extreme cases will effectively turn the Predator into The Unfettered, because, to him "nothing else matters." Failure to protect the Prey will cause anguish to the Predator, especially if he has to watch. The younger/ more helpless the Prey, the more extreme, the more violent the Predator will be in protecting him, up to full on Papa Wolf or Mama Bear. A Predator is almost by definition badass, but an Unfettered Predator lacking fear and not even caring if he lives or dies is both unpredicitable and scary. This trope seldom or never involves a sexual attraction/ relationship between Predator and Prey, and the Predator may not even like the Prey. Predators tend to be solitary hunters, but a Predator turned Protector will both accept and solicit assistance in protecting the Prey. The conclusive proof that that the trope applies is change: the Predator is unable to return to his former ways at the end.
- Schindler's List: An extreme example heavily based on real events during WWII. Oskar Schindler started as a businessman and Nazi Party member in Nazi Germany, manufacturing war material. When he discovered that his Jewish employees were slated for death, he did everything in his power to save as many as he could. At the end of the movie, what should have been his moment of triumph for saving more than 1000 Jews instead saw him breaking down in sorrow and shame that he didn't save more, that he still had anything (money or possessions) to his name (by which he could have saved more).
- Harry Potter: Snape, as a Death Eater, found to his horror that his own actions marked the woman he loved (Lily Evans Potter) for death, causing a heel face turn. His actions were not enough to save her, so despite hating her son, Harry Potter (whose father, James Potter, had made Snape miserable at Hogwarts), Snape acted through the entire series to save Harry from all harm.
- Twilight: The Cullens, a coven of vampires, are 'vegetarians' (do not drink human blood) due to choice and a treaty with the local werewolves and even act to protect a human (Bella), thus fitting (if loosely) under this trope.
- In An American Tail, Tiger (a cat) and Fievel Mousekewitz (a mouse) had become friends. In the sequel, An American Tail: Fieval Goes West, Tiger chose the role of a Protector, directly opposing other cats (all more badass than Tiger) to save Fievel, his family, and the other mice in town. To do so, Tiger took a level by learning how to be a dog. In the film's climax, Tiger tried to stop the killing of all the town's mice, and what started out as a last stand turned into a curb-stomp battle as first the training kicked in, then Tiger entered unstoppable rage and became a one-cat army. This is a milder form of the trope since Tiger never sought to harm Fievel (or apparently any other mouse), but cats are predators of mice, and Tiger opposed other cats to protect mice, so the trope definitely applies.
- In Monsters, Inc., Sullley (a monster whose job is scaring children) chose to be a Protector to Boo (a human child). His objective was to protect Boo and return her to her home and family. His reactions upon "seeing" Boo crushed was a Shout Out to Feed the Kitty (below). In the film's climax, Sulley went full Papa Bear to protect Boo. He even said "Nothing else matters."
- In Feed the Kitty (a classic Chuck Jones short), Marc Anthony (a bulldog) sought to protect Pussyfoot (a kitten). When it appeared Pussyfoot was killed, Marc Anthony suffered grievously. In Feline Frame-Up and Cat Feud (two additional shorts involving these characters), Marc Anthony went Papa Wolf to protect Pussyfoot from another cat, proving that his protection extended only to Pussyfoot. In these cases, tropers may reasonably argue whether Pussyfoot was a Morality Pet only instead.
- Ice Age: Diego (a saber-toothed cat) helped Manny (a mammoth) and his 'herd' to return a human child to his father, despite having been ordered to deliver the child to Diego's pride to be eaten. Although Diego had no particular love for the child, he did act as a Protector, though more out of loyalty to Manny's 'herd' than anything else.