Ok so this is an idea I've had for a while. It's called the Companionverse.

Near the dawn of humanity, a small comet struck in modern-day Indonesia, triggering the volcanic super-eruption that was Toba. While the eruption managed to wipe out 90% of the human race, nearly causing it to go extinct, it’s globe-encircling plume carried the comet's dust, depositing it in small concentrations all over the globe.
Within that dust was a magic virus, by sheer chance, could only affect mammals and avians. The virus caused the humans to mutate, their heads vanishing into their body, giving them rudimentary sight and hearing, but no smell, speech, and an inability to eat solid foods. They instinctively reached for their animal companions, semi-tame wolfdogs at the time, for comfort. The animal’s heads came off easily, their body sitting obediently.
Upon placing the canine’s head on their own, the humans had full sight, could hear like a dog, and more importantly, could eat solid foods. The animal’s jaw structure reshaped, allowing human speech, its head and body rescaling to that of a human neck.
From that point on, humans required an animal companion to survive. It would be found through trial and error that the animal’s body was needed, with its head allowing the same effect on it, and that the death of the body would mean the death of the head, and in many cases, the linked human from the sheer trauma of having their other half severed.
A human and its companion share a psychic bond. When one is hurt, the other feels it. They always have a rough idea on where each other is, and find it very difficult if not impossible to be more than a short distance away from each other.

Upon being born, a human will have already formed a loose bond with its companion twin, linking the two together. What forms this bond is still very much unknown, and is akin to how twins say they can feel each other in our world.
A bond can also form between couples, in this case four individuals rather than two, allowing either couple to remove or even use the other’s head, whereas typically the only ones capable of swapping the head between a human and their companion are the individuals themselves.
This type of bond only forms between couples that care for each other enough that their spirits form a connection not unlike that of human to companion.

Earth developed differently as a result, although still somewhat ending up where we are today, albeit with a few major diffences on account of the massive change to humanity.

More details will come out as I figure them out. I do have a discord server where I post stuff for this world.

These two meanwhile are named Ion (human) and Ajax (horse). You will see more of them in the future hopefully.

Original concept pic drawn by  cervelet

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