I also thought that was nonsense. I remember some module had ostrich riders, and I was willing to accept it for the sake of fantasy. "But a real life ostrich couldn't really support a human" I thought to myself.
However, before responding to OP, I did a quick Google search. People apparently do ride ostriches.
Should be noted, however, that while they can apparently support a human in jeans in a t-shirt; I'm pretty sure that a typical "we never bother with encumbrance rules" adventurer would break their back.