Apocalypse is more than his Horsemen. War and Death and Famine and Pestilence are all power players but Apocalypse has one ultimate goal: godhood. Over the years, he has pursued this goal with gusto, accumulating objects of power, worshippers, children and reincarnations.
Death Seeds supplant the abilities of the Horsemen of Death, infecting hosts and activating based on differing criteria to create mayhem. This most famously happened with Archangel, who was infected by Apocalypse while Archangel was a Horseman. After Apocalypse’s death, Archangel transformed into a reincarnation of Apocalypse. To combat this, Archangel was stabbed with a Life Seed, leading to his rebirth as Angel, minus his memories. A possible side effect of this is that Angel possesses more than just flight, and has been shown to cure people by touch. The psychological effects are even more drastic: Angel becomes convinced that he is an actual angel of God, enacting the will of God. Rick Remender, the writer behind most of the modern incarnations of the X-Force, has hinted that there are seeds for War, Famine, and Pestilence (although no writer has delved into those ideas quite yet).
Clan Akkaba are the worshippers of Apocalypse. Apocalypse was a member of the original Clan Akkaba in ancient Egypt, and one of the clan’s last survivors. He later warped the clan’s ideals and manipulated their tendency toward isolationism to his own means. They have served him ever since, even helping him battle Dracula sometime around the Victorian era. Historically, most members do not possess the X-Gene, but once Scarlet Witch muttered the famous “No More Mutants” line, the membership of Clan Akkaba swelled with former mutants. They joined thinking Apocalypse could restore their lost abilities, including future Horsemen, like Gambit. The members of this clan typically serve as rank and file canon fodder when the X-Force battle Apocalypse.
The Apocalypse Twins, Uriel and Eimin, are the two children of Archangel and Ichisumi. Together, they seek to free mutant-kind from the shadow of humanity. They are constantly clashing with Kang the Conqueror, who at one point travelled through time and stole them as children to raise as his own. Kang convinced the twins that they were the shepherds of the mutant race, and their destiny was to lead all mutants off of Earth. To accomplish this they trick Scarlet Witch into teleporting every mutant off the planet, conveniently leaving Kang with a mutant-free planet to rule with ease. However, questioning Kang’s orders and searching for the truth behind their childhood abduction, the twins decide to destroy the Earth after removing the mutants. In order to protect themselves, they appoint four Horsemen of Death to serve them. Daken, Banshee, the Sentry, and Grim Reaper were all given the title to assist in their Kill Bill-level rampage against Kang. The Avengers Unity Squad, led by Havok, do eventually end up beating the Twins thanks to time travel shenanigans, allowing Earth to survive.
Evan Sabahnur, also known as Kid Apocalypse, is the latest fail-safe of Apocalypse. Evan is the reincarnated form of Apocalypse, this time as a young mutant. He is taken in by Principal Wolverine (ha!) of The Jean Grey School, with hopes that he will not become as dangerous as Apocalypse was. In flash-forwards it is shown that Evan does eventually become Apocalypse, but these scenes are typically short, so it is unclear if he leaves destruction in his wake or chooses another path. His story is currently being told in All-New X-Men and he is one of the key players in the currently ongoing Apocalypse Wars story line.
So, as it turns out, becoming a supreme ruler and godhead can be a complicated business. You must have a devout group of followers, some goons for muscle, and a ton of back up plans. Once all of those elements are in place, you should be set to rule the world. Unless the X-Force is on your tail. Or Wolverine. Or James McAvoy.