Half-giants have not been tested on horses yet, though they also have not been explicity restricted. To be honest, I do not know what will happen. However, the horse models we have are all we'll have. There won't be any new (larger) ones added for half-giant sized creatures/PCs. In reality, about the only appropriately-sized horse would be a Nightmare, and well...rideable nightmares just won't be in. They're too high-spirited.
