How silly of me. All this talk of giant flightless birds and I forgot the one that most people do not even know about.
No, that is not an ostrich. It is a ratite, yes, but rheas (genus Rhea) are birds of a different color. They are much smaller than the ostrich and range over most, if not all, of South America. Despite the presence of mammalian predators, rheas are very common birds throughout most of their range.
The rhea is one of the smaller ratites, second only to the kiwi. This is, of course, relative to the other ratites -ostriches, emus, and cassowaries are bigger, but the males of the Greater Rhea can still get around 5 feet in height. The American/Greater Rhea (Rhea americana) is still the heaviest bird in America.
As one would expect, the people of South America use the rhea for meat. Insofar as I know, they do not ride them...yet.
Give it time. (I swear this is the last chocobo joke.)