Donaldson's attestation and discharge paper should have survived at Kew, Rimmington's was raised in Capetown on the 12th of October 1899, it ceased to be known as Rimmington's on the 21st of September 1901 and the new title was formally adopted just six days later on the 27th, via Army Order 414.
It was effectively disbanded in Kimberley on the 30th of June 1902.
You really would not be likely to enlist in this particular unit if you could not ride a horse, the ability to ride well was a basic requirement throughout the whole SAMIF and this unit in particular.
Because of the date Donaldson joined I would say you should look at WO126/145-163 as well as WO126/35-38 for any surviving paper.
His medal and clasps were issued from WO100/244, there is a note on the supplementary roll in that volume for his South Africa 1901 and 1902 clasps, which appear to have been issued to the Adjutant General New Zealand Forces on the 24th of August 1908.
Donaldson99 wrote: According 100to family lore my grandfather William Henry Donaldson was unable to ride a horse and thus was unable to join a NZ unit to fight in the Boer War. So he and a friend sailed independantly to SA and joined up locally. He is recorded as a member of Rimmington's Guides.
The story makes some sense however he was already in the NZ army serving in an artillery unit prior to this and continued serving with this unit on his return. It would seem he took leave of absence.
We have a photo of him wearing a hat with the leopard skin band. He appears in the list of members of Rimmington's Guides so there is no question he was there.
I also have a shell case which he said came from the last round shot at the end of the war. Mostly likely there are many other "last round cases". The diameter of the open mouth of the case is about 58mm.
So we a have an artillery soldier that couldn't ride enlisted in what appears to me to be a light mounted unit.
So I'm interested in finding out if Rimminton's had any artillery units and if not why was Henry a member of this unit. Was he seconded to another unit. Are there any service records available?