The basics are, I believe, 5 levels of Paladin to get access to a special mount, followed by a level of Beastmaster to get an Animal Companion. Then you take Devoted Tracker, which lets you designate your AC as your Special Mount, and go into Wild Plains Outrider. WPO allows you to add your WPO level to your effective Druid or Paladin level to determine the abilities of your Special Mount or Animal Companion... but thanks to Devoted Tracker they're one and the same, so the argument goes that it advances both. That's the basics of it, what comes afterwards really depends on the build. There's a build with Ashworm Dragoon and one with some PrC that gives you a Griffon. The most famous build is probably the Strongheart Halfling that takes Dragon Steed to ride a 27 HD Wyrmling Gold Dragon.
EDIT: More Halfling-specific, Halfling Outrider would seem to explicitly add its class level to your effective Paladin, Druid, and Ranger level.