This is a map I made for the Map of the Fortnight contest on AlternateHistory.com.
The basic premise is that the Quaternary Extinction Event, which killed off most of North America's unique megafauna in our timeline, is less severe in its effects. Several species survive in small numbers, before regaining their numbers once environmental pressures abate. The now-agricultural Native Americans experiment with domestication early, starting with small, llama-like camelids (which are bred for wool and meat), Equus Scotti (the American horse - too small and weak to ride, but a good pack animal nonetheless), and eventually the Camelops.
The Camelops is large and strong enough to ride, being seven feet tall at the shoulder (larger than any modern camel), and it's not long before they're heavily integrated into the society of many Native American groups. They're used by farmers as draft animals, by merchants for carrying their wares (as well as a form of transport), and eventually by soldiers.
Soldiers mounted on Camelops found it too tall to effectively attack foot soldiers from its back, so its main use is as a mobile archer platform. Its size and strength allow it to carry two riders with ease - sometimes with one directing the camel and the other shooting, sometimes with both shooting and one using their feet and legs to direct the camel.
Overtime the use of a rideable animal allows Native Americans to develop large empires. The Hundred Provinces (they use a base 20 counting system, so it should technically be "The Four Hundred Provinces" but the term is used poetically rather than literally) or the Kawek Empire ("Kawek" is derived from the Kawakahe - the Heron Clan - who ruled the empire at the time of the first European contact) - is perhaps the greatest of these.
Founded in roughly 500VA ("Vulgaris Aerae", Latin for "Common Era") the Kawek Empire rapidly expanded across central North America - sometimes conquering those who stand in their way, but more often allying with them and drawing them into the empire through sustained trade and diplomacy.
The Kawek Empire is notable for its many feats of engineering. Its society is based around clans, with each clan having a specific role - among them clans of labourers, who work to maintain the empire's roads and bridges; who level hills and mountains to make fields of golden corn; who raise huge, square and circular mounds upon which are built the palaces of the powerful, and the temples of the gods.
The above map is from a textbook detailing the Kawek Empire's history, showing the extent of the Empire and its allies at the time of their first contact with Europe in 1602VA ("Vulgaris Aerae", Latin for "Common Era" - it's the same as CE and AD). Columbus' voyage sailed off as it did in our timeline, but it never returned, delaying European discovery of the Americas until the 1600s, when an enterprising Frenchman with nothing to lose convinces the King to fund a fateful expedition across the Atlantic, and into the unknown.
Modern history of the Kawek Empire remembers their rulers at the time of this first contact as being part of the "Iron II Dynasty", so called because they ruled over a fledging iron age in the Empire, with iron rapidly becoming the most used metal for things such as luxury items, weapons, and armour. The Iron II Dynasty would maintain their hold on power until the final fall of the Empire in the 1800s, when Europe's technological edge finally became too much for them.
If you have any questions about the Kawek Empire or relevant allohistory please feel free to ask - I've thought about this quite a bit, but I've left some bits out to try and cut down the length of this description.