South Africa has a few pony-like horses used by the locals in *Free State and *Transvaal before the Boer War. Regardless, though it is my belief that, given time and a little prompting from the locals, it is easy to managing a horse breed adapting to the conditions of the Central Plateau in South Africa (which was packed end to end with grass eating animals).
Sheep were introduced to Southern Africa sometime between 200 BC and 500 AD. No one really knows how, the current theory seems to be: the Bantu brought them.
FYI There have been horses in Southern Africa since Europeans arrived--though not before, so that goes back to 1652 and van Riebeeck at least. The problem is they do not thrive. Apart from Tsetse fly in the North and rinderpest there is the problem of African Horse-Sickness which still exists and breaks out every few years. With good care and maintainence, horses can--and of course, do exist--today some of the finest bloodstock in the world is raised in the Natal Midlands and the Western Cape. But that is artificial. The animals used by the Boers that you refer to were relatively cosseted and were Kaapse Boerperd, a breed that was specifically created by Horse experts over 150 years. The early imports at the Cape rapidly became inbred and useless, Persian and Spanish bloodlines were introduced resulting in the Boerperd, which, while hardy and able to survive better on poor rations and harsh climate, can rarely survive without human intervention. An offshoot is the Basotho and Noitgedacht Pony, a runty creature descended from horses acquired by the Sotho tribes-they are not really well cared for, but in the Lesotho highlands and the Drakensberg there is appropriate natural fodder.
If, say, before 1850, you wished to travel from the Cape to modern day Kimberley your prime method of transport was by Ox-Waggon and the Oxen were an Afrikaander (Nguni) hybrid breed as European breeds would also not survive. If you did go on horse-back, you took your horsefeed with you or acquired it from vendors en route.
Re: the lack of fodder. The land between the Cape and the Highveld is Karoo--a semi desert, the nutritional qualities of the plants are very low and while native species have adapted, imported species rally struggle. For example, one can raise sheep in the Karoo today. It takes an acre of watered land to support one sheep, yet the same area could support 100 times the meat weight in indigenous game.
As for the "grasslands of the Central Plateau(?)", there is grass and grass. Any South African gardner will tell you that his biggest problem is native grasses (I think that there are a couple of hundred varieties and many are poisonous to domestic animals), with the exception of Buffalo, Signal and Nile grass few are good for fodder and the ones that are are not endemic. The diet of a Blesbok is somewhat different from a Palamino. All of which is one answer to why no indigenous horse evolved!
People have been crossing Zebra with horses and donkey's for 250 years in order to obtain a disease, drought and environment-resistant mount (lots of people have them as pets), but the result has always been a sterile, dwarf oddity. Maybe our Punics, with an extra 1500 years or striking a lucky gene could do better.
Re: The Fat-tailed sheep of the Cape.
While I don't think that the Merino Register was open in 500AD, I will not dispute the date. The first Europeans making Landfall in the Cape in the 15thC traded with the Khoi for 'fat-tailed sheep' (one piece of copper wire the length of the animal per sheep was the going rate).
Now here is the question. You will not find a sheep, wild or domesticated, South of Ethiopia in pre-European Africa (predators, climate) and the species in the Cape (some still exist) is a direct descendent, not of the Abyssian variety, but of the breed common in the Middle East and the Levant that has its roots WAY back.
Discounting the lost tribe of the Israelites theory, my question is:---
WHERE THE F**K did these animals come from?
Our friend's Punic expdition perhaps?