"I have his horses."
Translation:Zȳhī annī eman.
Why is "jahī" not a possible solution. The class of the possesor is not known, so both could be used!
In theory yes, but since human beings are almost always lunar, it will also almost always be "Zȳhī". The only exceptions might be if it refers to a Targaryen (which, in HV, is terrestrial).
If the sentence would be "I have its horses", it would be more ambiguous. In this sentence both versions would be equally possible, but in this specific case I think "Zȳhī" would be very very strongly preferred.