Ok I came up with a special Feat:
Stud Mount [general, fighter]
Prerequisites: Str 16+, Male Humanoid, Owned by Stud Rider
This Feat allows the weight of the Stud Rider female or a similar saddled burden to be treated as 1/4 for capacity weight. So a Stud Rider weighing 140 lbs. would effectively be 35 lbs. for encumberance. Also add +2 to Prowess (specific person) checks when used if a specific Stud Rider is listed by name.
Berzerker Stud [fighter]
Prerequisites: Stud Mount, Barbarian Level 1.
This feat allows the Stud to be a ruthless melee fighter when so enraged. First the Rage duration is doubled. Second they gain an extra 1d12 hit points. Third they get an additional +2 to Prowess with their owner.
As for sources I'm sticking to core 3.5 and what I get on line but that does include psionics, bloodlines, variant classes and such to a fair degree.
As for the society I'm setting it up similar to Sparta with women in charge. So slave women often are used for breeding males and only lucky males get to breed with a free citizen woman- Stud Mounts are generally well regarded by their riders. But generally women are expected to have women lovers and most older women take a younger one upon age 16 as a lover to guide her in life- usually the age difference is 16 years or more. Men are either citizen males as in have some use in domestic management, overseeing farms and the like but very little rank. Women run everything. Among the respected trades as in will get people alot of respect are:
Fighters (Amazon warrior are always respected and the top of the food chain pretty much)
Wizards/Sorcerors (wizardry is a difficult craft and fairly rare and sorcery is an uncommon and wonderous talent- and so respected)
Clerics/Druids (faith carries alot of weight)
Bards (as the other magical professions plus hold religious posts and are usually wonderful companions)
Rogues/Rangers (sneaky wilderness scouts are generally well regarded)
Nobles (directly in the line of succession and well treated, if they are good leaders)
Men on the other hand are more limited:
Commoners (your rank and file male is a laborer only age may grant some respect like an old farmer wise at their work)
Experts (women can learn serious crafts like making weapons and healing but men can learn "women's crafts" like pottery and weaving again age is the key- can also be kept men basicly skilled househusbands)
Fighters (Stud Mounts and only they can fight and favor Spiked Gauntlets and unarmed combat- they can train in Monk as well and for them Spiked Gauntlet is a Monk Weapon- Also they can stack Medium armor.)
Barbarians (Studs not ridden can be chained and used as EXPENDABLE shock troops basically they get worked into a frenzy and get sent in well before the main force- only their owners can control them and are known to be vigourous lovers.)
There are NO Paladins. Prestige classes are also non-existant.
For Riders mounted combat is big most warriors will know Mounted Archery, Use Composite Short Bows, Spears and Swords and Up to Medium Shields.
Dress for men is usually nude or a loincloth with sandals and maybe a warm cloak. Women also wear very little a small waistskirt, a simple tunic revealing one breast and a cloak with sandals are common. Hair ornamentation and jewelry are common for women often shared with a lover to match in spots like both wearing the same bracelet.