One thing I've been thinking about is, could Llamas (or their equivalent cousin) be bred to be "stronger, bigger, faster", and if so, in how long of a timeframe? I know a llama can only carry maybe 25% of what a horse can (roughly 80 pounds vs. almost 300 pounds for a horse), and probably pull (if it had a cart) - I'm guessing here, I've searched... - maybe 2,000 - 3,000 pounds per llama (horses can pull 6 times their weight).
Also, if elk/moose could be domesticated (I know, I've read some of what Diamond wrote about domestication - but, maybe steal the kinder, gentler moose and elk?

), things like antlers, fur, and intolerance to heat need to be dealt with. If a moose didn't have antlers, would it be more agreesive?