Llamas As A Combat Animal

Okay, so, I know it's extremely implausible that the Incas could have ever trained Llamas to be mounts or anything besides pack animals, but I was reading up on Llamas today, and man.

Let's just say that something happens and the Inca managed to train Llamas as combat animals and mounts, with the biology they have now. How well would Llamas work in combat, against people and against horses when the Spanish show up?
 
I can't imagine llamas holding up against horses. The llama isn't quite as tall as horse which puts the rider at a disadvantage and I'm not sure how long a llama could hold a human rider.
 
Llamas are too small to be mounts for anyone but children. You'd need a larger species of llama to do it. The only ones large enough were in North America and died out 10,000 years ago.
 
Animals can only go up to a certain size before health problems kick in. With llama's they MAY become large enough for small adults but it will take a lot of time and effort.
Also important is how limited the population of llama's is. Generally the larger the population the more you can tweak things through breeding and mutations. Thats why dogs are so varied, we have always had a lot of them, so there's more chances for good mutations to pop up and be breed into the population.
With llama's they're stuck in a single region, its a big region but its pretty uniform. Also while they were widespread in that region, they were never really raised in huge numbers. So the genetic diversity, while healthy would require a lot of luck to get the right mutation to increase in size. And once that is reached it would take careful breeding to make sure it continues.
Possible, but unlikely.
 
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