This information is based on that found in the Genertela book in the Glorantha boxed set, plus more recent data from King of Sartar, from Tales of the Reaching Moon, and from the Digest.
(from the Genertela book)
Originally the Pentans were a nomadic people who herded horses and worshipped the sun, who they called Kargzant. After being defeated by the Darra Happans and their allies in 230 at the battle of Argentium Thri'ile they were driven into the wastes of Pent, and there learnt to ride their horses as well as herding them, and to hunt for survival. The cult of Kargzant was functionally identical to that of Yelm, although neither cult likes to admit this. A few changes were introduced after the Pentans moved into the wastes: in particular, while they still revere the concept of divine blood, inheritance of leadership is not necessarily to the eldest son but to the most worthy candidate of the bloodline.
The deities worshipped by the Pentans (other than Kargzant) can be found by studying those of the Grazers as listed in King of Sartar. I'm assuming that the worship of La-Ungariant, of whom the Feathered Horse Queen is an avatar, is not known to Pentans. (The Grazer founder, Yanasdros, "took the Horse Goddess as his wife" as part of his founding of the tribe in a new land). The main women's goddess is Arandalya, the horse goddess. Orest, earth mother, would also be followed.
Pentan society is divided into age groups as well as by gender, according to the traditional Yelmic system. Males are Riders when they turn 14, Warriors at 23, Leaders at 43, Chiefs at 50 and Elders at 55+. Women are Riders from 14 until married, Mothers until all their children are 14, (they will now be about 40 themselves), then Teachers until they become Elders at 50+.
Each group has its own deity. Male Riders follow Dastal the Hunter. Female Riders follow Sun Daughter (similar to Yelorna). Warriors follow Jardan the Golden Bow. Leaders follow Henird, Chiefs follow either Karzgant directly or Polestar, and Elders follow Josad. Adult women are less rigidly divided, and may follow Arandalya or Orest as they prefer.
(Some of these names may have changed between Grazers and Pentans, but I don't know of alternatives).
Between 1375 and 1460 the Pent nomads occupied the Lunar Empire. They were led by the most famous Lunar enemy, Sheng Seleris, who brought down the Empire, plundered the Moon, and killed the Emperor. The Red Emperor did not re-appear for 16 years, and then started to drive the nomads out.
For more details of how the Empire did this, see Nikk Effingham's cult of Yara Aranis. For a very Darra Happan view-point, reading "The Fortunate Succession" may also be of interest.
At the Battle of the Nights of Horror in 1506, the nomads were decisively crushed. They fled back to Pent, and it is said that the survivors each had a hundred wives the next year but many of the women and children died in the winter because they had no men to help them. Since then the Pentans have abided by an agreement to send all their red-haired children as hostages to the Lunar Empire once they reach the age of 13.
After this massive set-back, the Pentans started looking for new ways, both for immediate survival and to improve their chances next time they fought the Lunars. Not all tribes chose to try the same changes, so now all the tribes differ from each other.
The Traditional tribes still ride and eat only horses. "Modern" tribes ride horses, but herd cows (or sometimes sheep or goats). These tribes generally have additional goddesses who care for the new herd animal. Names vary, but they will be functionally identical to Eiritha (as is Arandalya, of course).
The use of wagons varies from tribe to tribe, depending on their desire for mobility and their ability to acquire the wood from which to build them. Where they are used, they are often brightly painted.
The sexual restrictions of the Pentans are generally not as severe as those of other sun cultists, though this again varies from tribe to tribe. Also, while the position of adult women is just as restricted as that of their solar (or Praxian) counterparts, unmarried girls are Riders, carrying out the same duties as the boys of the same age. Some tribes actually require that each girl and boy kills an enemy before being allowed to marry, but many require them to prove themselves in less suicidal ways - the "enemy" may be a predator such as a wolf, or a feat such as winning a Chadash match may count.
For those who dislike the gender restrictions that still apply at adult level, the "Women Warriors" is a new tribe for those who find their traditional social roles unpalatable.
Pentans take a pride in their ability to endure pain and hardship: and, therefore, in their ability to inflict pain upon others. (Giving an enemy a chance to show his courage as he dies is regarded as an honour: a clean kill would assume he was a coward). Sheng himself was perhaps the greatest exponent of this viewpoint - to outsiders, he was known as the greatest torturer and mass murderer in Gloranthan history.
(For a really gruesome account of Sheng's early life, take a look at some stories by Martin Laurie. Warning: I mean really gruesome!)
To a large extent Pentans worship the same deities as before the time of Sheng, but with additions that vary from one tribe to another. All tribes worship spirits in addition to the solar pantheon. Ancestral spirits are common, and many also treat deities of other pantheons as elder spirits. In particular, some tribes have taken to worship of the storm pantheon: West King (or Rain) Wind, North War Wind, South Rage (or Dry) Wind, East Sting Wind. These spirits are remarkably similar to Orlanth, Humakt, Storm Bull and Gagarth.
It is rumoured that some Pentans follow Ikadz the Torturer, but this may be just propaganda.
Raven (a shapechanger aspect of Trickster) is seen as a benign agent of change, having introduced his four brothers to Pent. Followers of Raven wear black and white striped clothing: the Pentan view of Zebras may perhaps be guessed from this!