Sure, movies can be copyrighted, but did you know monsters can, too? And even a particular version of a monster? Zombies were around before George Romero's 1968 movie Night of the Living Dead, but he put his own spin on the idea. His version -shuffling corpses that menace humans in order to eat their flesh- became the zombies we see in movies like 28 Days Later, World War Z, and Zombieland, and TV shows like Z Nation and The Walking Dead. It all has to do with copyright.
YouTuber kaptainkristian (previously at Neatorama) explains how and why Romero's zombies became ubiquitous under the law of public domain. -via Tastefully Offensive