Earth-17 is a weird medieval post-apocalyptic world with armored knights riding albino mutant horses.Those are the original Atomic Knights, from the "Strange Adventures" series. And those "mutant horses" are actually giant Dalmations.
Originally the Knights were survivors of "The Great Atomic War of 1986", but the global catastrophe was later vaguely described as "The Great Disaster". Kamandi's adventures were also retconned as part of this world. See:
http://www.toonopedia.com/atomic_k.htm Thank you, so much DCU history so little money!
ConfirmedEarth 2 = Golden Age
Earth 3 = Crime Syndicate of Amerika
Earth 4 = Charlton
Earth 5 = Captain Marvel & Family
Earth 10 = Freedom Fighters
Earth 17 = Atomic Knights
Earth 22 = Kingdom Come
Earth 50 = Wildstorm
UnsureEarth 1 or Earth 52 = New Earth
Seen but not yet designatedTangent
Dark Knight Returns