Around a thousand years after ancient Egyptian culture really got going, Osiris and Isis started to appear in the stories and monuments of ancient Egyptians, with their son, who is named for his uncle, Horus the Elder. So nobody would get confused, they called the little guy Horus son of Isis or Horus the Younger (even though both gods are kings, have eye problems, and pick on Seth). Eventually, Horus the Elder became a sky god and a warrior, and Horus the Younger became associated with kingship.
Horus the Younger made a name for himself as a special god associated with the pharaohs themselves. As Isis and Osiris became more popular with the people, so did their son. Eventually, people got confused about which Horus was which—especially because there were a couple dozen other gods running around calling themselves Horus, too. (Horus the Elder says imitation is the sincerest form of flattery.)
A papyrus that tells the story of the 70-year battle between Horus the Younger and Seth is written on the back of another papyrus filled with boring accounting numbers. It's a powerful, PG-13 rated story that probably got told around campfires. Now if only someone had invented papyrus comics…
Horus the Younger is one of the most popular subjects for Late Period Egyptian art. They loved to cast bronze statues of him, especially in his full-hawk form. Beautiful paintings in papyri show him visiting his father Osiris, and showing the souls of dead people who made it past their final judgment into the Hall of the Two Truths.
The Ptolemies, late kings of Egypt who weren't Egyptian, wanted to make Horus the Younger easier for non-Egyptians to understand. Just like they made his father Osiris into Serapis, they created a new form for Horus the Younger—Harpocrates, a naked boy holding one finger to his lips. In hieroglyphs, this is just the glyph for the word "child," but the Ptolemies couldn't read glyphs. As a result, they thought it meant silence, like we still do today. Harpocrates became a god of silence and secrets, even though Horus the Younger talks just fine.
Horus the Younger followed Isis to Rome, where he was still worshipped. Since Roman warriors didn't wear the funny robes of Isis priests, they found a new way to dress him: as a Roman legionnaire in a full suit of armor.
Ancient Egyptian medical texts tell doctors who want to heal an injured eye to pour milk over it, just like Hathor does to Horus the Younger after Seth rips out his eyes. In some hieroglyphic texts, it says specifically that it should be the milk that Isis fed to Horus the Younger when he was a baby. Eventually, the Romans changed this to the "milk of any woman who has had a male child." Gross as it sounds, through the next two millennia and up through the Victorian Age, doctors and medical books all over Europe continued to advise using mother's milk to heal sore or damaged eyes.
Horus the Younger's Eye appears in the Great Seal of the United States, above an Egyptian Pyramid. This seal was created by the first U.S. government back in 1782, but it's still being used today—get a dollar bill and look at the back, and you'll see Horus staring back at you!