'No mounts for you, off you go walkies,' says the developer.
'You're just pulling a Guild Wars 2,' respond the playerbase (a game which launched without mounts before, eventually, adding them).
So the game addresses this through some lore: a note you find called 'The Animals Revolt' that's headlined 'A Notice of Concern to Carters and Porters' written by 'your friendly local doctor.'
It has to be the silliest explanation on the planet.
"After a recent spate of back injuries, the doctors of Aeternum feel compelled to issue this warning to anyone who transports goods between settlements:
"Please remember that there are NO beasts of burden on Aeternum. No horse or donkey will pull your cart, carry your pack, or tolerate a rider. All efforts to domesticate or re-domesticate these animals have led only to injury and a lot of swearing.
"Therefore it's important to transport ONLY as many goods as your own back can bear. Do not overburden your packs or carts. This is not the Old World. In Aeternum, we must all pull our own weight.
"Your joints will thank you!"
So... the horses have revolted? This seems like it should be more reflected in the world than it is (see also: all of New World's lore)—like perhaps we should have spent the first hour fighting graminivorous quadrupeds rather than mini Dr. Manhattans?
Either way, now we know: you can't have a mount in New World because, apparently, they just attack people. Check back in a few months for the retcon!