Not that you necessarily want to do this, but one of the cool things about Halfling Outrider is that as written it's the only path that will increase both animal companion and paladin's mount. People like to make supermount builds using the Devoted Tracker feat (lets you count your mount as an animal companion) and stacking up on bonus hit dice from multiple classes. You end up with a mount with epic feats and better combat abilities than you.
so akin to the companion familiar of the arcane heirophant? hmm, its interesting, i like it a lot! i still would want to be halfling driud sub level for all of its 1st lev abilities. i would also (if doing a druid build) the ability to cast the bull's strenghth series of spells (cats grace etc). but they could come later.
how about halfing (racial sub levels 1st+5th)druid 5/pally 5/halfling outrider 10?
This would grant undersized wildshape 2/day, enhanced link (+4 to ride checks, no penalty for riding bareback), 3rd lev spells and spontaneous casting of jump, spider climb, and protect vs energy. also would get 1st level pally spells a coupla auras, lay on hands etc.
add to this a mount-companion equal to a 18th level druid (with natural bond feat) and a paladin mount level of 15 (from a 1st lev druids companion) stacking. the only thing i could think to add to this is a level of beastmaster to erase the effective druid level penalty of the animal companion.
thanks for the idea! a lawful neutral arbitor of nature it would have just enough spells to buff itself and mount a coupla healing spells to be self sufficient. it would be able to use heavy armor (if its non-metalic) and a tower shield even! imagine a defensive-riding halfling outrider's defense with a tower shield! plus the ability to use spider climb on the (medium) mount will make one VERY versatile in a dungeon i think.
wonder if a level of arcane heirophant can be added in there to make the animal a companion familiar special mount (all abilities stacking?) need to re-read that section.
in my humble O though a pally of nature would have to be lawful neutral, as i said an arbitor of nature. i would even think it would get different auras, smite...chaotic? nature?o, and maybe turn/rebuke nature (your choice on creation but have to stick with it).
Of course, the resulting character is weaker without the mount than yours would likely be, and the halfling druid levels are tempting. Your character probably ends up more well rounded, though with a somewhat weaker mount.
By the way, remember that Devoted Tracker doesn't make your animal companion your mount, it makes your mount your animal companion...that means it's chosen from the mount list, not the animal companion list. This loses you some of the cool druid options, but for a feat you can get a wyrmling gold dragon, which is pretty kickass.
I believe the standard way of doing it is something along the lines of Ranger 4/Paladin 5/Beastmaster 1/Halfling Outrider 10, with natural bond thrown in, for 15th level paladin and 19th level druid. Otherwise you have to go to the more interesting sources (animal companion wizard 2, cleric 3, beastmaster 1, prestige paladin 2, halfling outrider 10, lesser bloodline 2 has (with natural bond) the mount of a 18th level paladin (young bronze dragon or very young gold dragon with an extra advancement, with a feat) and the animal companion of a 23rd level druid)
I'd replace the Ranger with Wild Plains Outrider if you can spare the feats (without Strongheart halfling such a build may have to go Pal6) Ranger lowers your animal companion level.
Edit: this is better: Strongheart Halfling Paladin 5/Beastmaster 2/Wild Plains Outrider 3/Halfling Outrider 10. Gets 18 druid and 18 paladin, so there is another level of advancement for a dragon, meaning that you can get a 26 HD wyrmling gold dragon that can command creatures of its kind