Sheep, Ovis aries, are the 'without-which-not' of the South Downs. They created the short turf which, after a couple of hundred years, can contain more flowering plant species in a square metre than any other habitat. If we were all vegetarian for the next three hundred years we would lose a significant portion of our flora! Once ploughed, however, the ecosystem is destroyed. The owner of Offham Down tried to plough his ancient grassland and got more than he bargained for. Amazingly after 'unploughing', it continued happily as before. So much of the Downs was ploughed during the Second World War that this unique habitat is now scarce, with most of it having some kind of protective designation.
The pond is a 'dew pond'. These are a feature of chalk grassland and were so called because it was believed they were filled by dew: sadly the more prosaic explanation that it's simply rain actually holds true. They were created by hollowing out the chalk and lining it with clay and straw. Their purpose was to provide water for the sheep so it's somewhat ironic to see them now fenced out.
Sheep really took off, so to speak, in medieval times when monasteries made millions out of the wool trade. In the last 100 years the Australians and New Zealanders have done much the same, although wool production in Australia has more than halved since 1990. They are in the same family, Bovidae, as other cloven hooved ruminants such as cattle, antelopes and goats. They started being domesticated from wild mouflons about 10 000 years ago and there are now over 200 recognised breeds. These are Suffolks if I'm not very much mistaken, and not the famous Southdowns bred locally by John Ellman of Glynde.