You know, instead of spiked horseshoes, we could do armor spikes on the barding, which sort of looks better in my mind's eye. Then we don't have to worry about the two-weapon fighting rule, and the base damage is 1d8. But you're right, the two hoofs seem like they'd be better anyway, and the armor spikes have the disadvantage of being martial weapons.
Personally, I'd just stretch the rules and give them 1d6 damage horseshoes without any attack penalties. They weapons are pretty useless with the RAW -4/-8 two-weapon attack penalty.
As for armour spikes, we'd also have to give the Houyhnhnm an Armour Proficiency feat in order to wear the armour the spikes are attached to, or it will suffer an even greater penalty.
We could note somewhere in the Weapon Nonproficiency SQ that Houyhnhnm can use use spiked gauntlets to increase their damage on hoof attacks without a two-weapon attack penalty? I fancy a base Houyhnhnm does not have any proficiency in armour or martial weapons (i.e. armour spikes), but only a few simple weapons.
Speaking of which, are there any other simple weapons a Houyhnhnm could be skilled in? If I remember my Swift correctly, they farmed crops so could know how to use sickles. Since they're cavalry it may sound like they should be skilled in lances, but a horse rearing onto its hind legs and charging into battle with a spear is too silly for my tastes.
Hold on, there is precedent for a monster making a normal iterative attack with an artificial weapon and then using a natural weapon as a secondary attack. A Houyhnhnm using armour spikes could become:
Full Attack: Armour spike +5 melee (1d8+3) and hoof +0 melee (1d4+1)
That still works out worse, with a -5 attack on the hoof and no increase in average damage if both attacks hit. Maybe if we say a Houyhnhnm counts armour spikes and gauntlets as natural weapons, which would make it:
Full Attack: Armour spike +5 melee (1d8+3) and armour spike +0 melee (1d8+1)
That's potentially an extra two points of damage if both attacks hit, but still isn't optimal except against low AC targets. I think that's excusable.
Alternatively, we could apply a rather liberal interpretation of "The Two-Weapon Fighting feat lessens the primary hand penalty by 2,
and the off-hand penalty by 6." so it cancels out the -5 secondary attack penalty to become:
Full Attack: Armour spike +5 melee (1d8+3) and armour spike +5 melee (1d8+1)
The TWF could be a racial bonus ability applied only to two weapon fighting with hoofs, gauntlets or armour spikes, analagous to a Ranger's TWF, or we could swap the Endurance or Run feat for Two Weapon Fighting.
That's two points extra damage on average (2d8+4 averages 13, 2d4+6 average 11), while a horse with spiked gauntlets would do the same damage average, but piercing (1d6+4 plus 1d6+1 = 2d6+4 averages 11).
I quite like that.
EDIT: If they get TWF as a bonus feat, then they can buy Improved TWF for an extra secondary attack, which I like the idea of. What out for those high level Horse Monks with Flurry of Blows!
I still fancy statting up a Houyhnhnm Warrior with armour spiked studded leather as a 'Militia Horse'. Make it 3rd level, so it gains an iterative attack and an extra +3 BAB:
Full Attack: Armour spike +8/+3 melee (1d8+3) and armour spike +8 melee (1d8+1)
(Assumes the TWF proficiency option, it can use its 3rd level feat to get TWF if we decide all Houyhnhnm don't have it, otherwise it would have another feat such as Weapon Focus (armour spikes) for an extra +1 to hit.
Sigh. We've really started to hit the complicated ones. I suppose the best weaponry for these is really the light crossbow. We could just go with two hoofs or a light xbow and forget the parenthetical weapons (could still do parenthetical armor).
EDIT: Complicated? What makes you think that?
