The legendary Namor the Sub-Mariner as his chosen Horseman. He may have been reinvented in Jonathan Hickman's current X-Men books, but Apocalypse is traditionally one of Marvel's most dangerous villains. Considering himself the first mutant, Apocalypse believes the doctrine of "survival of the fittest" should be applied to humanity - and, of course, to mutantkind.
Apocalypse has access to the technology of the alien Celestials, and he routinely uses this to transform standard mutants into the so-called "first mutant," and at one point he even recruited the Hulk as War. But, surprisingly, the potential Horseman Apocalypse seemed most interested in was actually Namor the Sub-Mariner.
The story is told in 1993's Namor The Sub-Mariner annual, in which Namor encounters a beautiful woman named Mimeyoshi. Never one to say no to an attractive woman, Namor allowed Mimeyoshi to seduce him, but she used their embrace as an opportunity to drug him instead. The Sub-Mariner resisted the drug, but found himself overpowered by Mimeyoshi, who proved to be something of a physical powerhouse. Although he was defeated, Namor was able to tear away her face, revealing she was nothing but an android.
Mimeyoshi had been created by Apocalypse, the culmination of years of manipulation in order to successfully get his hands on Namor. It's easy to forget that Namor is actually an Atlantean mutant, which meant Apocalypse was understandably fascinated by him. He had noted Namor's warlike nature and his prodigious power, and resolved to transform the monarch of Atlantis into his mightiest Horseman. The idea had become all the more pressing given Apocalypse's frequent conflict with the X-Men, and their successful redemption of Angel, who Apocalypse had transformed into Death. As Apocalypse noted, Namor is not the type for camaraderie, which meant nobody would be likely to rescue him.
Apocalypse had made one major miscalculation, however. As part of the conversion process, he exposed the awakening Namor to seawater - and that had a pronounced effect on the Sub-Mariner. Namor's powers are tied to the oceans, and the saltwater replenished his energy, allowing him to break free. Worse still, this particular base was an underwater lab, and Namor delighted in bringing it down around his ears. "We're on the ocean floor," Apocalypse roared in fury. "You fool! Do you wish to bring the Pacific down on us?" After a heartbeat, even Apocalypse had to it the absurdity of his statement. "Actually you probably do..." he conceded. And that was exactly what Namor did. Years of planning were ruined because of one foolish mistake - and Namor the Sub-Mariner never became a Horseman of Apocalypse.
