The term "Conan the Barbarian" never occurs in the original Howard stories. Instead Conan tends to be called "Conan the Cimmerian", "Conan of Cimmeria", etc. However, "Conan the Barbarian" was in use at least as far back as 1954 (as a book title) and probably earlier. And of course, one of the Marvel Comics series and the first movie use it.
Doing It for the Art / Money, Dear Boy: The last few stories written by Howard include "Beyond the Black River," "The Black Stranger," and "Red Nails," novella-length stories that are not particularly commercial ("The Black Stranger" wasn't even published during Howard's lifetime) but are considered some of his best and the clearest examples of the themes he was exploring. But in the same time period he also wrote "The Man-Eaters of Zamboula," a decidedly more commercial, less thought-provoking tale with a scene (the nude Zabibi avoiding the snakes) that seems to have been constructed specifically for one of Margaret Brundage's Weird Tales cover illustrations (which it did indeed get).
Franchise Killer: The second film, Conan the Destroyer, undid all the lingering good will from John Milius' first film. It wasn't even the negative critical reception of the second movie that killed the franchise; it actually was successful at the box office, just less than the first one. This prompted the production of the Red Sonja movie - now that was a big flop. Schwarzenegger called it "the worst movie I've ever made" and may have had a hand in making him move on to other projects - when a third Conan movie was considered, he was already busy with Predator.
Name's the Same: Contemporary writer Clark Ashton Smith has a prehistoric fantasy setting called Hyperborea. However, Clark's setting is dark comedy set in prehistoric Greenland. Howard's Hyborian Age is roughly set in what will become Eurasia and focuses on heroic fantasy.
Outlived Its Creator:The series started in 1932 and the creator Robert E. Howard commited suicide in 1936. Many other writers continued writing short stories and novels into the 21st century.
Science Marches On: The concept of continental drift was new and little-understood at the time Howard was writing, so the idea that the European landmass could have been vastly different merely thousands of years ago wasn't as unlikely as it seems to today's reader. We also know a great deal more today about anthropology and ancestry than what was incorporated into the stories.
Spiritual Successor: The video game franchise Golden Axe takes lots of inspiration from Conan the Barbarian. One character is even an Expy of Conan.
Around 2001 and 2002, John Milius (the man who wrote and directed the 1982 film) wrote a script titled King Conan: Crown of Iron that was intended to be a true sequel to the original film (as opposed to Conan the Destroyer, which was considered a disappointment to most fans of the original). The film entered the pre-production stages and Schwarzenegger was set to star in it. However, when Schwarzenegger was elected Governer of California in 2003 the project was put on indefinite hiatus and eventually canceled.
Roy originally planned an adaptation of Conan of the Isles, the "last" Conan story, to be told over three issues of Conan the Barbarian Annual. However, his departure from Marvel meant that the story wouldn't be fully told in comic form until it was published as Marvel Graphic Novel #42.
A similar situation happened with the Comic-Book Adaptation of Hour of the Dragon, the only full Conan novel by Robert E. Howard. It was originally going to be posted in all 5 issues of Giant-Size Conan, but the slipping sales on that title meant the novel's adaptation was completed in Savage Sword. Giant-Size Conan #5 was a reprint of the Conan/Elric crossover, with a new cover by Jack Kirby.
Speaking of MGN, Roy had plans for a new story about Belit to be published in the title in 1993. The title only had one issue that year (a The Punisher/Black Widow crossover), and was cancelled that year. The story was eventually published in parts in Conan the Savage, a Spiritual Successor to Savage Sword of Conan.
Out of fear that they wouldn't get the rights to Conan, Roy and Barry created a similar character named Starr the Slayer. He looked exactly like Conan but with a different coloration and it seemed his stories would have featured him as king only.