Alright, here's an idea I had for my PFS GM credit dump character with a race boon. I wouldn't be playing him until a high-ish level.

Nagaji (+2 STR/CHA, -2 INT)

17 STR (+1 at 4)
13 DEX (+1 at 8)
14 CON
10 INT
8 WIS
16 CHA

Barb1: Power Attack
Barb2: Moment of Clarity
Oracle1: Bonded Mount, Boon Companion
Oracle2: +1 STR
Oracle3: Animal Affinity, Nature's Whisper
Oracle4:
RageProphet1: Extra Rage
RageProphet2: +1 DEX
RageProphet3: Weapon Focus (Greatsword)
MammothRider1: Huge Mount
MammothRider2: ???
MammothRider3:

I haven't worked out the details of my mount, but the general idea is to run around attacking for huge damage with a greatsword, while my mount does the same. I'd have up to 3rd level spells for buffing, although that means I'm losing some great 4th level buff spells by taking the barb/rage prophet levels. Any ideas or suggestions?

the general idea is to run around attacking for huge damage with a greatsword, while my mount does the same.

When you say "the same", I assume you mean "attack for huge damage", not "attack for huge damage with a greatsword"? Because the latter would be a no-no in PFS, if I'm not mistaken.

You assume correctly.

Hmm was just making a barbarian build with a single dip into nature oracle - I concluded the barbarian didn't add much to the build...

oracle1/barbarian 11/Mammoth rider 3 -

All the barbarian barbarian did for the mount was a gore attack, greater rage, and +10 foot movement (posisble a few passive rage powers).

And the barbarian him self used all he's feats to pull it off - boon compagion, mounted combat, ride by attack, spirited charge...
Things any char could have done...

Hmm, that's an interesting point. Now I'm wondering if going Fighter (Dragoon) 1/Nature Oracle 8 might be a better idea. I'd get an extra feat, and if I give up Weapon Focus and Extra Rage, I can get Spirited Charge and all its prereqs by the time I take any levels in Mammoth Rider. Plus, I'd get a 4th level spell (Blessing of Fervor!), another revelation, and a few more FCBs. That sounds like the way to go now.

I'd only be down one druid level on my mount, so Boon Companion seems like a waste. Are there any traits that could make up that one level?

I would just eat the level. There's also the option to go without any fighter levels at all, although you'd have to switch to morningstar or longspear.

Yeah, the Fighter level gets me 2 bonus feats to help my mounted combat, another point of BAB, martial weapon proficiency, and a couple extra HP. I think that's worth losing one level toward my companion. I couldn't find a trait for it on the PFSRD, but if there was one, it would be perfect.

How about a straight mounted fury barbarian build with boon companion.
Raging HUGE animal companions..

Instead of a raging mount, I'd rather buff it with Bull's Strength and Blessing of Fervor. Overall, I like the spellcasting levels better.

Going oracle of battle gives you weapon focus and improved critical for free. Any PFS character who relies on a large mount is going to be totally useless in some modules. And hindered in a high proportion of the others.

That's true, but it seems a shame to pass up such awesomeness, just because it won't always work. I can always read the description of the scenario, and guess if a mount will be usable or not.

Anyway, here's what I'm thinking now:

Cavalier (Emissary) 1/Nature Oracle 8/ Mammoth Rider X

Cavalier1: Power Attack, Mounted Combat
Oracle1: Bonded Mount
Oracle2: Animal Affinity
Oracle3: Nature's Whispers
Oracle4: Ride-by Attack
Oracle5:
Oracle6: Spirited Charge
Oracle7: Natural Divination
Oracle8:
Mammoth1: Huge Steed
Mammoth2: Lightning Reflexes

Using the level of Cavalier gets me everything the level of Fighter would, except the Skill Focus (Ride), but keeps my effective druid level maxed out. Even if I am separated from my mount, I'll be able to cast up to 4th level spells, and have good melee damage and AC on my own.

As far as the mount goes, I'll probably use the Axe Beak boon until I get to Mammoth Rider, at which point I'll switch to either the Megaloceros (a giant elk, which is relatively fast, and has 3 attacks but lower STR and natural armor), or the Rhinoceros (which has only one attack, but better damage and natural armor).

Unless you have something special, Nature's mystery oracles are specifically limited to horses for medium sized characters.

nature oracle bonded mount has the same animal companion choices as the cavalier.
if the axe beak boon would work for a cavalier, it will likely work for nature oracle as well.
anyways, AC classes explicitly stack for purposes of level, so he would be able to use both class' mount scaling anyways.
mammoth rider explicitly says it replaces other class' AC's, so once you are a mammoth rider you may not choose from other class' mount options, albeit if you have a feat/boon allowing you more options, that technically still applies. (HUGE AXEBEAKS!!!)
i think that technically, a nature oracle/mammoth rider's mammoth mounts would not benefit from the higher INT since that is tied directly to the nature oracle bonded mount. but paladins' abilities to call mount to side, apply celestial template, SR, to their mount ISN'T directly tied to 'paladin's mount' so would apply to mammoth rider mounts.

Man, huge axebeaks would be awesome. And soooo scary.

I looked at the Axe Beak as a huge mount, but it just wouldn't have the damage output as some of the others listed for the Mammoth Rider class. Lower STR, lower armor, only one attack, and with a lower damage die.

Also, I've never played with a mount or animal companion before. What kind of feats and skills should I invest in?

you might not have the feats for it but animal companions w/teamwork feats can be great (if you also have the feat) - great for Cavaliers in particular.

Animal Companion feats can be complicated but very very strong - the feats can make a companion quite effective in combat. And occasionally give them utility outside of combat.

if it only has one natural attack, vital strike is great,
and can still be worthwhile even if it has several attacks.
taking improved/greater grapple or trip or bullrush or over-run
can synergize with abilities like grab, trample, etc.
i would say that readying a partial charge is a very useful ability,
so picking up power attack/improved overrun in order to take rhino charge could be a good bet.
feat-buffing feats are a good idea (iron will), overall you just want survivability
with being able to have 1 or 2 schticks that it can solidly contribute to combat.
toughness could be good as well. lunge is nice also.

i would focus on perception, acrobatics (jumping), and swim and climb. swim DCs don't scale that high, so you just need enough to cover the Armor Penalty and Encumbrance and pass DCs reliably. sometimes you can take 10 but sometimes you can't, so being able to pass most DCs even on a roll of 5 is a good baseline to aim for. intimidate might be a good one depending on feats, or sense motive could also be good if that matches the role and character you want the animal to have ("What's that Smashy? He's Lying/ Is a Bad Man? OK..."). Especially if the animal has Scent, maxing out Survival (or just increasing it somewhat) makes alot of sense. There's probably others I'm forgettign :-)

yeah, axebeaks are great for the trip aspect, and looking awesome, but that's about it :-)