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There: one horse, no rider. We know there were at least three horses—one for each rider going in different directions—but it seems one returned, and without a rider. It’s hard not to imagine that this is the end of the road for Rakharo, which is why Irri is likely so grief-stricken.
This is, of course, a change from the novels… probably motivated by the fact that Elyes Gabel was cast in World War Z, which filmed in Scotland during the summer of the production.
To speculate a bit further, if this is indeed Rakharo’s horse coming back riderless… I suspect that once the other bloodriders return, Daenerys will decide to travel in the direction the horse came from, to see if they can find Rakharo or what happened to him… and that leads them smack dab into Qarth. Will it turn out that the Qartheen murdered Rakharo?