Ferdain is a temperate planet of rolling, hilly grassland, dotted with occasional valleys. It is the primary horse-breeding center of the galaxy, distributing these ancient mounts that have served mankind for millenia across all reaches of the galaxy to be employed in either martial or recreational pursuits.
It is host to a series of large-scale underground facilities run by the Magos Biologis of the Adeptus Mechanicus - the Equestrarium. They are used to engineer horses tailor-made to fit in every theatre of war via the use of vitaegenic cloning vats, selecting valuable traits from mounts throughout the Imperium (IE, those found on death world or high-gravity planets). These can range from unstable creatures requiring a constant cocktail of chemicals in order to function, all the way to beasts that can consume anything barring plasteel and live for months on their fat reserves. It is from these facilities that the original genetic template for the mounts of the Death Riders of Krieg originated.
The planet is divided between completely rural countryside and sprawling cities, dashed in cobblecrete and filled with expensive statues and architecture. The lower class tend to work as stablehands and ranch owners, or eventually move on to the city to take jobs in training warhorses and breaking them in. They live out a simple and reasonably well-fed (if hard working) existence, often having an acre or two of farmland in order to supplement their earnings from the horses. Ranchers generally buy genetic "off-cuts" from the city markets, breaking them in and then making the trip to hopefully shift them onto a merchant ship the next time one comes to the nearest city.
The upper classes tend to be foreign - mostly Praetorian merchants, many of which have holiday homes on the planet. They live in incredible homes and estates, often populated with many mansions and far more rooms than they will ever need. They will sometimes own a ranch on the side so that they may practice noble sports with others of their stature, usually on horseback. The upper class pride themselves on being gentlemen, often scorning the lower class for breaking one of the major taboos of the planet - a man is only as good as his hat.
Generally speaking, the entire planet is proficient in horse riding, with most hobbies revolving around the creatures. Horse races are a particular favourite for commoners to drag themselves into the upper class - though it takes a lot of money and status to persuade their new would-be-equals that they are worth anything if they are born of the lower class.
Adults not proficient in horse-riding (mostly off-worlders) are looked upon in complete confusion and pity by Ferdainians, often treated as though they have some form of disability.
The Magos Biologis are an unknown, even to the native population. It can be assumed that they live out a comfortable experience in their subterranean complexes, though even if they didn't one would imagine there is no place they'd rather be.
The far outer-reaches of the countryside are plagued with feral Ork tribes with a taste for the local horses. They have developed a form of guerrilla warfare that employs swift strikes via racing across the numerous grassy planes towards undefended small settlements on boar-like creatures, forcing many of the lower class to learn how to handle a weapon early to fend them off.
Originally teeming with feral Orks, Ferdain was freed from Ork control by eight regiments of the Praetorian Guard. The planet was swiftly awarded to the General of the campaign, who had it immediately turned into a private retreat for the noblemen of Preatoria to get away from the sprawling, scum-filled hives of the planet.
Ferdain used for this purpose for many years, being passed on eventually to the General's grandson. By this point, tribal feral Ork forces had begun to spring up on the planet, threatening the property of those who had joined his grandfather's scheme and thus threatening his own reputation. The threat was not deemed sufficient enough for Imperial Guard retaliation, and as the planet was his own property, it was deemed his responsibility to defend it.
Oddly enough, it was a senior Magos Biologist of the Adeptus Mechanicus who approached him with a proposition. His solution was simple - he would have Skitarii dispatched to destroy the uprising and install planetary defences up to scratch with the current Adeptus Mechanicus. In return for this seemingly genuine request, he had the Governor pay him a hefty tithe for his services, and claimed wide areas of seemingly useless land as the Mechanicus' property. Seeing little other option, the Governor reluctantly agreed to his terms.
The Orks were swiftly put down by the elite Skitarii, and the Servitors brought on-world swiftly began digging out foundations for huge underground complexes. When completed, the facilities were silent for almost a year. When they finally roared into life, the Magos Biologis instantly began working on their purpose here - humanity was not the only thing that could be improved by the glory of the Machine God. If they were to transcend mere flesh and limitation, so too, would their beasts of old. Cavalry would be a viable tactic once again, being able to operate with little-to-no supply lines, no Techpriests required constantly on-hand to administer prayer and litanies, and all the armour of a light tank by installing a naturally-growing organ inside rapidly-accelerated clones.
And so they got to work, obtaining a huge stock of animals from throughout the galaxy. Horses were deemed the most suitable for use due to their old role as stalwart war beasts. Amongst their first improvements came the sub-dermal armour implants, suddenly making mounted warfare that much more viable. Regiments that had once simply requisitioned a unit of Rough Riders when they needed a unit to charge and outflank suddenly began opting into the scheme. Over the next few hundred years, the Equestrarium became a galactic distribution center, constantly shipping out mounts for new issue or replacement as though it were simply a shipment of lasguns.
The Ferdain Mustangs are only a relatively recent development; the planet reached a population to have to provide tithes to the Imperium's war effort that would not simply be covered by their other industries any longer. This was co-ordinated by the nobles, and as such ceremony took place over marshal practicality.
Each of the regiments is run and owned by nobles who have purchased a commissioned officer's rank, only some of which have much military experience. The NCO's, which are generally the highest ranks commoners can achieve, tend to fill these gaps by using their heads to accommodate for any shortsightedness on their superior's part (with said superior usually taking the credit if it goes right or having said NCO court-marshalled if it goes awry). The lucky ones will be under the command of an old Praetorian officer who has grown bored in his retirement. Others will be forced to follow the whims of a rich, young nobleman who has little to no clue just what he has signed on for.
The Mustangs use a form of Service Dress as their standard uniform, accompanied by a peaked cap. Sergeants tend to be slightly more garish, usually with shoulder-boards and embroidery to denote them being more important than the standard rank-and-file.
Commissioned Officers tend to have all form of eccentric styles to their uniform, often attempting to outdo each other with materials such as the furs of off-world carnivores being fastened upon their uniforms. All soldiers are expected to keep their appearances in top condition at all times - a Guardsman who keeps losing his hat without sufficient reason will generally be shot for insubordination.
In cases where marriage is not possible, lower-tier noblemen are often sworn into the service of more prominent families in order to forge alliances and agreements. If the family in question has a stake in a regiment of the Mustangs, the nobleman in question becomes a Knight - having a horse commissioned directly from the Equestrium to fit his needs and tastes and being given a suit of carapace armour with a filter-fitted helmet and emblazoned with his sponsor's family crest and effects. These Knights tend to look down on the average trooper with disdain, knowing full well that they can never achieve the same status that they have. The Guardsmen are generally respectful of the Knights to their faces, but hate them and covet the protection that they are given off-hand.
When serving on in the PDF or in some form of social function, Mustangs wear the Dress Uniform that they keep in their saddlebags, which is expected to be well-maintained and looked after even during hellish campaigns. It is said that the veterans of the Mustangs have more polish on their boots than they do leather.
The standard weapon of every Guardsman is a sword and laspistol, both of which are trained with heavily while the Guardsman is on horseback. The Mustangs are also expected to engage in swordplay amongst themselves, as it is seen as both a means of increasing their combat-effectiveness while also conditioning them into gentlemen through a noble activity. Others are sometimes issued with folding-stock lasguns, made popular by Drop-Trooper forces.
All horses are made up of a myriad of different sources, sometimes being altered to suit current military direction (such as creating horses with higher heat tolerance for desert campaigns). Most rank-and-file mounts however, incorporate a large amount of genetic stock from Attillan breeds. They are all fitted with sub-dermal armour, mostly concentrated at the front to counter the age-old tactic of shooting the horse rather than the rider. All soldiers are issued a kit in their saddlebag to aid in treating lasbolt burns and bullet wounds caused by this. The heavier cavalry of the Knights employ flak harnesses over this, also being issued with gasmasks in the same vogue as their riders. The Dragoons use horses with slightly less sub dermal implants than most, but this is to take advantage of the horses' enhanced strength no longer being used to support its own weight as much, allowing them to reach much faster speeds than conventional cavalry would achieve.
All forces are proficient in mounted combat, including officers. When an entire regiment is deployed, it is rare to see any man on foot. Though this can be seen with bemusement by those of other forces of the Imperial Guard, none can deny the sheer power of an Officer-led cavalry charge with the fury of the Emperor behind them, sweeping upon their enemy as a massed wave.
Women are forbidden from ever being in the military, and the very notion of a woman fighting in a war is obscene to even those of the lowest status.