I’m thinking of replacing horses with an extinct llama from North and South America called Hemiauchenia. Hemiauchenia is about the size of a camel and is more adapted for running than any other camelid, but could you even ride them comfortably in the same way you’d ride a horse? If these animals are uncomfortable to ride, I don’t think they’d exactly be a nice candidate to replace the horse’s domestic niche. They’re pretty much a huge guanaco, so I’d image they run like one, or possibly they’d run more like a camel?
Llamas and alpakas are used to carry some 20-ish kg up and down all over the mountains.
Make you llama big enough to carry some 100 kg (human + gears) and you have your people carrying-quadripede. You do not need to have a 5x scaled llama, something like 1.5x or 2x should do the trick.
Riding a llama is fraught with difficulty. The spines of llamas are not strong enough to support an adult. For an adult to ride a large llama a specially made saddle, that evenly distributed the weight of the rider along the back of the llama would need to be used.
Llamas have been used throughout their history as pack animals. Llamas typically aren't used for riding, but some are bred for this purpose. For the most part, it is recommended that only children under 50 lbs should ride llamas. Most llamas cannot withstand the weight of an adult directly on their spine. While they are bred for carrying cargo, the weight of their packs are well distributed. Anyone riding a llama should make sure it has a well made pack saddle