[quote=redPANDA]
That is a seriously badass bird. I want one.Theoretically, would their necks hold the weight of a human?
[/quote]

The saddle would be harnessed in such a way to keep weight off the neck. I doubt you could ride them bareback which is why it may have never caught on with the Aztecs. Also the Spanish have been breeding them as mounts since the 1500s so the ones used by people would be of a sturdier frame than the wild variety.