Desert giants are…yeah, you guessed it. Giants. That live in the desert. Oh wait, I’m repeating myself from yesterday, aren't I?
Obviously desert giants are masters at traveling over sand, and they put even the usual desert dervishes to shame when it comes to their scimitar skills. But that’s all price-of-entry, right? So what else is interesting about them?
First they're not very magical—much more on the mundane end of the spectrum than more mythic giants (for instance, the cloud giants that sometimes sail over their lands). That means the clerics found in larger desert giant tribes are likely well respected, even at low levels. Surprisingly, they also do not have the natural affinity for animals that most giants have. Certainly, the nobles are likely to have eagles or other large birds, the way a human lord would keep falcons. But instead of the usual cave bears or hell hounds, desert giants prefer humanoid slaves. Also, even the most benevolent giants are rarely creatures of their word, making the lawful neutral desert giants a rarity among their kind (and slightly less likely to roast you alive than the lawful evil fire giants).
On a side note, not every campaign world needs every giant. I can easily imagine campaigns where desert giants are the only giants in the setting. Encountering one of these 15-foot-tall men might be just one more reason going across the dunes is a dangerous proposition: “The South is a land of giants!” Or if you're doing a One Thousand and One Nights-style campaign, you might have a single legendary city where all the inhabitants are giants—a nice change from the usual cavalcade of jann and rocs.
Anyway, on to the adventure seeds!
Adventurers share the hospitality of a desert giant tribe as they travel across the wastes. Things get complicated when the tribe’s cleric passes away during the night. Given the rigors of desert life, especially the need for healing and water purification, it is custom that no desert giant tribe should be without a divine caster…and the next caster who appears is duty-bound to serve a clan in need. The apologetic but firm sheik shrugs that the adventuring company’s cleric is short but that she’ll do. Meanwhile the entire tribe begins sharpening their blades in case the party attempts to defy custom.
Desert giants may come from the desert…but they don't have to stay there. Their nimbleness and skill with blades make them excellent pirates, provided accommodations are made aboard ship for their size. Many a corsair has swooped down upon what he thought was a simple dhow from afar, only to discover up close that the ship’s apparent small size was an optical illusion created by its 15-foot crew—who are now bearing down to board in turn.
Manticore Jack is haunted by a ghost train. Years ago the desert giant guide lost a lightning rail work crew to a gnoll attack and a freak tornado that swept them both dwarves and gnolls into the sky. Now he relives that night in his sleep—a haunt of the train and its crew follows him wherever he goes, manifesting during storms and on nights the moon is just right. Ending the haunt means finding what happened to the train or convincing Manticore Jack that the deaths weren't his fault. But a side effect of the haunting is that while in its throes Jack can't tell friend from foe. The afflicted desert giant may attack adventurers, thinking they are the hated gnolls come back to finish their raid.
—Pathfinder Bestiary 3 128
Obviously, there’s no reason Manticore Jack can’t be the keffiyeh-wearing giant of the Bestiary 3, but for a more American Old West feel give him cavalry sabers and maybe a few levels in an appropriate hunter/tracker class/archetype.