If I make a mammoth riding character, how should melee combat work? Would my character need a reach weapon to attack things adjacent to the mammoth he's riding? Would those who are adjacent to the mammoth need reach weapons to reach the rider?
I'm not interested in the rules so much as I'm interested in advice.
Thanks to the guy with the face for asking this question.
For simplicity, assume that you share your mount's space during combat.
So basically you can attack any square as if you occupied any of your mounts individual squares or be attacked as if you occupied all of them.
As for advice, not really sure there is any to be had.
Maybe invest in trip and combat reflexes to keep enemies from closing to close.
Claxon has the right of it. Because you share your mount's space (for simplicity purposes, of course), this means that creatures can hit you as if you were the mounted creature. This still means that the mount has its natural reach (in other words, can hit 15 feet), and that your Reach still counts as normal (meaning a Lance allows you to threaten from afar, but not adjacent).
Most Reach builds tend to go for the Trip maneuver, as it allows you to continue utilizing your ability to make AoOs when others can't hit you. As they have to spend an action to stand up (which provokes), you could spend your AoO to trip (via Combat Reflexes), Full Attack on your turn (and then 5-foot back), have them stand up, try to get to you again, and repeat until dead.
It fails if you're fighting other creatures who can hit you normally, or if they're just too damn big to trip; there is a size limit to your Tripping (If it's more than 1 size larger, you just fail). It also doesn't work too well on enemies with a lot of legs.
To that end, Dirty Tricks may also be useful, since you could use a Lance to poke an enemy's eyes out, making them Blind, or a jab to the gut may cause an enemy to double-take making them Staggered.
There's also feats that allow you to do Dirty Tricks while making a Full Attack, so you can still deal damage if you go this route, and your debilitations last longer (and are more effective). The big problem here then results in you yourself not dealing the damage (until the later levels), and your Mount doing the clean-up. Which isn't bad, since it basically has just as much Reach as you.
If you/he is asking for advice on how to run a character on a huge mount (in a less simple fashion than the rules suggest), I might have the rider choose a side that they're riding closer to and threaten/attack that side (i.e. left/right/front/back, maybe corners). I wouldn't want to get much more complicated than that. Though, I'd probably give something to reach weapon users to threaten more squares along a side, and maybe a couple squares 10 ft out from the mount, too.
If you're looking for a bit more realism.
Another 'realism' house rule would be that if you're sitting high up on top of a huge creature, you can hit anything on the ground unless you have a reach weapon, in which case you can attack anything within 5 feet of your mount. But you might then need to impose further rules for how you interact with flying creatures, and whether creatures on the ground need anything special to attack you.
That's not a bad idea. Maybe just could the PC as 10-15 feet in the air?
Yeah, I think that's a good judgement call; if you're riding a huge mount like a mammoth you need a reach weapon to attack opponents adjacent to the mammoth on the ground, and they need a reach weapon to attack you.
I don't think it rises to the level of house rule. It's just a part of the game that calls like this need to be made--despite the rules. That's why I reposted this guys question (a good question) in the Advice section rather than the House Rule section.