There were irregular mounted units raised outside Natal - and there were Natal volunteer units which had existed prior to the Zulu War e.g. the Natal Carbineers had been formed as far back as 1855, the Alexandra Mounted Rifles in 1865. The Imperial Irregulars included men from the Cape and Transvaal and a sprinkling of foreign nationals. Derogatory phrases used in describing Colonial forces, such as 'flotsam and jetsam', are inaccurate when applied to the Natal volunteer units. These men, many of them born in Britain or of British settler stock, were of good standing in their communities, well-respected farmers, business and professional men, civic leaders etc.
The volunteers operated on a personalised system of recruitment: a man was elected to membership by other members of the corps if they considered him suitable. This produced a highly-cohesive and committed group. It wasn't unusual to find several men of one family serving in the same unit and members would be from within a distinct geographical area.
Tracking down written records of an ancestor in a Colonial unit isn't easy. Although the Soldier's Discharge Papers at TNA include copies of documents for men who took their discharge outside the British Isles, this would apply to British Army regulars, not to men in the Colonial regiments. According to one source - and nobody seems to be able to confirm this for me, which may mean it is true - Colonial service records were pulped for paper during World War I.
As a first step towards finding out more about your Colonial soldier check the 1877-9 South Africa Medal Roll. If the man was a recipient the Roll will give you the name of his unit. A list of the Colonial units entitled to the campaign medal with bar '1879' as well as those units who received the medal with the bar, can be accessed at http://samilitaryhistory.org/vol044fm.html
At TNA, Medal Rolls for the Colonial units are in WO 100/48, WO 100/49 and WO 100/50.
The family historian should bear in mind that many of the lists, rolls and other sources used when tracing military ancestors may contain errors and omissions and that the spelling of surnames may vary.