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?