Shiva will be the final primal that drops a pony, so I've read in some live letter somewhere. I suspect you will need that mount to get the special gift. Don't get your hopes up too high. The reward will likely be a a title like, "Mounter of All Ponies" or "Imfamous Pony Mounter of Eorzea". Maybe it will be a hat that changes skins to every pony, if you are lucky.

Ideally, I'd love to see a chariot mount that uses all the primal ponies; that would be epic.