John Calverley was a white trader, he along with James Rorke were Nominated by Hamu to negotiqate a surrender with Wood. They went to Woods camp to do so, Calverley had Coghills horse ( reputably a spare) and a MH rifle from iSandlwana. He must have a had a fare explanation for those items as he stayed with Wood and fought and died at Hlobane.
No doubt Cohill would have ahad a spare horse so that is emminently possible.
A certain well known author has state pretty emphatically, without proving, that Coghill wasnt riding his own horse but 'grabbed' the nearest one available, not his own. That doesnt ring true as Coghill was incapacitated with his knee injury and would, in order to attend to his duties, had to have been mounted for much of the battle sequence.
Higginson in reporting the sequence of events at the Drift stated unequivacally that Cogjills horse was killed in a volley of rifle fire.
There were also rumours that Calverley blacked his face and pretended to be a zulu at isandlwana. Takes more than a touch of coal black to transform into a zulu.