From the basic Swords and Sorcery fantasy up through the Weird West/Wild West eras of fantasy, the noble steed is a constant companion. He is not abandoned until Modern, and futuristic settings, and even then the horse finds ways to sneak back into the game. There is a certain mystique to the image of a knight riding astride his destrier with lance lowered, or the man at arms swinging his sword in a killing arc.
So what would you world be like without horses?
Additional Ideas (4)
The horse has played a role in the history of Falhath, though the role of the cavalry and the knight are overshadowed by the heavy infantry that dominate Falhathian warfare. While the loss of the horse would be a moderate blow to the Falhathians, it would be a severe blow to the Ankarans and the other inhabitants of the central plains. Where Falhathians tend goats and other relatively small domestic animals, the plainsfolk are hunters of highland antelope and bison, and would have never founded their semi-nomadic civilizations.
Their efforts in war would be doubly wounded as the cavalry forms the core of the army. The northern and western parts of the continent would be more overrun with goblins and other fell races as humanity would like the speed of the horse and it sinnate advantage in melee battles, and Falhath would have spent its time faced not with barbarians and border raiders, but facing hordes of genocidal orcs, goblins, and other monsters who would have been held in check by the plains folk.
In this vein, the Via Mortuus would have been supplanted by the Old Faith, and the roots of the Trinity would have been cut long before they rooted.
The Horse is one of the most important domestications a culture could have.
Domesticated in central Asia around five thousand years ago, the horse was instrumental to the development of Eurasian civilization. Unlike most other large mammals, it was not farmed for its meat, milk or hides. Instead, the horse was harnessed solely for its incredible strength – to pull plows, vehicles, and most significantly, to carry humans themselves.
European civilization ascended to power on the backs of horses. Without horses, the evolution of complex European economies and trading networks would have been unthinkable. Most significantly, the horse transformed the art of war. From the earliest horse-drawn chariots of the Hittite empire, to the bareback cavalrymen of Attila the Hun, the warhorse has become synonymous with Eurasian military success. Those who had horse warriors "won" until the comming of age of the firearm.
Spanish horses were instrumental in the conquest of the New World. Neither the Aztec nor the Inca had ever seen humans riding animals before; the psychological impact of mounted troops was tremendous. The indigenous people simple froze in response to the rider and horse. It was so alien to their experience that they did not understand it or its implications. The invaders were able to cover tremendous distances and fighting techniques that were useful against a warrior were pointless against a mounted one. Sure foot warriors could have good success against cavalry if they stood their ground and grouped appropriately. But since the Horse had been extinct in the new world for 8000 years, they had no experience to draw upon. (Interesting history note: The "modern horse" actually came from genetic stock that had migrated back again from the new world to the old across the Bering Straights.)
On Arth the horse has a strange history.
The Elventi, the first culture, never domesticated any animals. Animals were more enculturated to Elventi presence, so predators seldom attacked them and other creatures ignored them.. those that feared or responded negatively to Elventi were quickly culled. So while the animals were "open range" they were impacted by the Elventi.
The Elventi are able to magically enhance their endurance, so moderate distance trips are not that much of a burden. The Elventi used magical gateways for long distance travel. It was faster by far than any means of travel. Never having experienced a riding beast, or even having that idea, simply hiking about was "the natural order of things".
The Humanti managed to domesticate a few animals before they were forced from SecondLand. Dog, Bovine (The Arthian "Cow" is more Buffalo like), and Horse, were all domesticated before the exodius. Only a few samples were brought with them to FirstLand. Since FirstLand did not have a good bovine or equestrine substitute, they Humanti did not domesticate anything locally. Eventually a small handful of creatures were important to FirstLand for their culture. The art of the horse was diminished, if not practically lost, over the Time of the Visitation.
Note: The Humanti gave this idea to the Elventi. Now the Elventi actively cull and modify the behavior of the animals in their areas... before it was kind of haphazard. The Cat is the only domesticated animal they have truly embraced.
Horse, while powerful in warfare, became a liability when fighting the Telepathically Controlling Wapti. Humantis had been fighting on horseback and chariot for quite a while before thier comming. The Wapti ability to control all lesser minds in an areas made combat on horseback more of a danger to the riders than the Wapti. (Note: The Wapti have a harder time controlling "dulled" domestic animals, though they can influence their actions. The same was said of Humanti.)
The art of fighting from horseback was nearly lost in the Exodius. Stangely enough it saved as an art, limitedly, by the remaining HighLanders. (Given their terrain, horses were and are problamatic). The speed the horse provided.. and its power.. was quickly rediscovered by the Returners (who really relearned the arts from HighLanders). The speed of transport freed Elventi power from being used simply for transportation and allowed it to be used for active combat. As long as horse minds were shielded, they were valuable tools. In fact, horses in SecondLand today still wear a crystal suspended over their forehead as part of their harnesses, mimicing the shields of old.
In ThirdLand, horse equipped Humanti were the primary colonists. Humanti fighting Humanti often used horse to their own adavantages (thus the Knights of the Villages). However, horses are mostly used for travel as Gates are only used for "Official Imperial Business" on ThirdLand.
Indigenous ThirdLanders on the Plains of ThirdLand already had the horse (or were centaurs), so they were adept at such combat. They do not have the innovations of the Stirrup or saddlehorn. They only use a Hackmore for control of the beast. However, generations upon generations of riding has given them great skill with their beasts.
MaskLanders came to the KnownWorld with the horse. They are not great riders, but they have long engrained experience with riding.
Coastal ForthLanders have the horse, though their local horses are poor specimens and not very advantagous. The N'geti find animals as equals rather than things to be ruled.
So, now that the summary is done... If horses went away? Since you can't have diseases on Arth, it would have to be some catostrophic failure of the bionet maintained by the pylons. Horses are an intergral part of the biome in the Plains of Antioch. Someone or thing would have to rewrite the reality matrix for those places for horses to go away. So the places would quickly become some place new and different.
Now if the biomes were altered in other places, there would be minimal impact. There are other burden beasts, including any number of furry folk, so they could be easily pulled out. Medium Distance travel is really all that they are regularly used for. Short distances are best covered by foot and Long distances are either by ship or gate. Loss of the horse would be an inconvience to people, but the world would go on.
On Kerren there are a limited number of surviving Terran Horses. Luckily for the colonists, dragons existed. Note: there are some Southers attempting to breed Runners into a riding beast. They are having more success with that than they are to breed a proper Kerrenese "Chicken". Of course, that is not a "lot".
One of the hard science components of Traveller was their rules that state: combustion functioning engines must be specially tuned to each world they are used on, as differences in O2 and inert gas levels as well as pressure make standard settings pointless. Add to this the limit of someworlds preventing fusion and atomic power plants, and the heinous limits Traveller put on electrical vehicles, there was no real useful vehicle you could take from world to world reliably. The design consideration was that you were always supposed to either just fly OR pick up local transport.
Many traveller players solved this problem with low berths and Horses. And these were not normal horses either, but TECH 10 horses with genetic enhancements which includes increased intelligence, strength of muscle and bone, increased endurance, superior immunity and healing systems, and a genetic advantage when placed in cold sleep. These horses could go anywhere right off the ship. Sure they were not "as fast" as some high tech items, but they could go places and be counted on for things that no standard vehicle can go. The best part, they were never limited by local law because they were "organic" and of a standard breed, so no special licensing was needed. With a low berth (cold sleep) you could keep a herd of them without having to carry too much in the way of feed and supplies.
So the world would not change without them, but it would be more inconvient.