So since it was determined officially here
http://paizo.com/paizo/faq/v5748nruor1fn#v5748eaic9qqn
that Druid and Cavalier levels stack for AC for determining the abilities of the animal when the animal is available on both classes' lists... I'm a little unsure about something regarding the Beast Rider archetype and available mounts.
Quote:
Medium beast riders can choose a camel or horse mount at 1st level. At 4th level, a Medium beast rider can also choose an allosaurus, ankylosaurus, arsinoitherium, aurochs, bison, brachiosaurus, elephant, glyptodon, hippopotamus, lion, mastodon, megaloceros, snapping turtle (giant), tiger, triceratops, or tyrannosaurus as his mount. Additional mounts might be available with GM approval.
The wording of this suggests to me that you have to have 4 levels in Beast Rider in order to have the available companion list open up to include lions and tigers.
So if I have a 6th level Druid, if I took a level in Beast Rider. I would be forced to have a different animal companion until I have 4 levels in Beast Rider in order to stack my effective Druid level for my Animal Companion.
So Lion Shaman 6/Beast Rider 1 would have to have a level 1 Horse or Camel, and a level 6 Lion. The Horse/Camel would level up to 3 as I take Beast Rider levels, until Lion Shaman 6/Beast Rider 4 when suddenly can stack my levels and have a level 10 Lion?
Assuming I'm reading the RAW correctly here (while considering the FAQ answer about effective Druid level stacking) the effective Druid level is NOT used to determine the available Animal Companions in this archetype, only the Beast Rider level. Which seems a bit off to me, having a 3 level gap where it doesn't stack and then it suddenly stacks after 4.
Mechanically it strikes me as odd, and thematically I'm entirely against it not stacking for the purpose of determining the available mounts. On one hand it kinda makes sense that you would have to specifically train yourself to be able to handle riding a more powerful animal... but at the same time at Druid level 7 the Lion would become large anyways, and mechanically I would be able to ride it comfortably without any levels as a Cavalier, so it isn't like thematically or mechanically I wouldn't gain the capability to ride a Lion without Cavalier levels (though I'd miss out on Cavalier abilities). Additionally if it did stack, the animal would still become large and I would be able to comfortably ride it. If it's supposed to be nothing more than a tax, I'd say missing out on spellcasting levels is already enough of a tax to gain the low level Cavalier abilities.
In a home campaign I would rule without hesitation that if counting effective Druid levels makes the animal available, and if effective Druid levels would stack should the animal be available... then it should stack. Largely because it makes no thematic sense that a Druid with such a close tie to a Lion companion, would suddenly obtain a horse (that the class feature dictates has a close bond with the character) then ditch it the moment 4 levels have passed... then the Lion suddenly becomes 4 levels more powerful.