You're a cleric of Erastil with the Animal Domain or a Sacred Huntsmaster Inquisitor of Abadar, and in both cases you take the Chivalry Inquisition.
In my view, so long as you pick a rideable mount from the mount list, this gives you a horse animal companion under an effective druid level of sacred huntsmaster level*2, or cleric level+(cleric level-3).
Supporting Rules text & FAQ;
If a character receives an animal companion from more than one source, her effective druid levels stack for the purposes of determining the statistics and abilities of the companion.
If the animal is on the cavalier mount list and on the list of animal companions for your other class, your cavalier and druid levels stack to determine the animal's abilities. If the animal is not on the cavalier mount list, the druid levels do not stack and you must have different animals (one an animal companion, one a cavalier mount). For example, if you are Medium druid and you choose a horse companion, levels in cavalier stack to determine the horse's abilities. If you are a Medium druid and you choose a bird companion, levels in cavalier do not stack to determine the bird's abilities, and you must choose a second creature to be your mount (or abandon the bird and select an animal companion you can use as a mount).
This same answer applies to multiclassed cavalier/rangers.
(Note that the design team discourages players from having more than one companion creature at a time, as those creatures tend to be much weaker than a single creature affected by these stacking rules, and add to the bookkeeping for playing that character.)
Question of RAW; You guys know of any rules text to the contrary? This is definitely abusing the rules, no question, but am I reading the rules incorrectly? Please avoid PFS specific rules, if you can, they don't apply to my GMs game. Although other people might benefit from knowing about them so bleh.
Into the Opinions: This obviously allows for a stronger than normal mount. Is that okay? How would you make the rules on this issue look, in an ideal world? If you put up a cap of effective druid level = character level, or perhaps = # of levels classes that grant AC's, Would you extend that to other classes' abilities as well? Would it apply to things that are normally supposed to break caps, like Orange Ioun Stones, Bracers of Smiting, the Huntsmaster feat, the Aasimar Oracle FCB and so on?
How would you make the rules on this issue look, in an ideal world?
They'd say you can't count the same level more than once no matter what you do. (Actually I think the rules do say that but I can't find it... would still be the right answer if they didn't.)
(Actually I think the rules do say that but I can't find it...
The only place I've ever seen such a rule is for the gestalt variant in 3.5e. I don't believe it exists in pathfinder, otherwise the Order of the Star Cavalier VMC trick wouldn't be legal, as it makes your channeling levels count half again.
It was actually a pretty big deal, as there were a number of things that allowed you to stack and sometimes restack your level for things, which stopped working when applying that variant rule. I recall a feat that added manifester level to caster level and vice versa, which could get out of hand when used with a psion/wizard/cerebremancer/mind mage build, for instance.