I've seen it come up a few times now, and it really bothers me. I get that, from our perspective, it makes sense - they're big birds that people ride like we do horses. But on Hydaelyn, chocobos are the baseline for mounts; not horses. To most people in Eorzea, horses are a rarity. There's the tribes of the Steppe over in the Far East that regularly use horses, there's Sleipnir, and there are PvP mounts that are horses, but your average joe is going to be far more familiar with a chocobo. Why would they call them horsebirds?