Found in Dorset, England and in southern Ireland, this short (about two feet high when fully grown), shaggy creature walks on two cloven-hoofed feet and feeds on grass. Its small arms end in four fingers. The porlock lives to guard horses and will be found with a herd or curled in the straw of stables, although it is so distrustful of humans that it is seldom seen (FB).
Porlock is a town located along the north coast of Devon, on the edge of Exmoor National Park. According to What's On Exmoor, "A large group of feral goats lived in this coastal area until about 1910 and their milk, skins and meat were traded at Porlock Weir..." Perhaps it is this bit of history that suggested the Porlock to Rowling.
The Porlock could be Rowling's nod to a local legend in the Exeter area of Devon, where she went to university. The legend is known as the Devil's Footprints:
"The Devil's Footprints is a name given to a phenomenon that occurred in February 1855 around the Exe Estuary in East Devon and South Devon, England. After a heavy snowfall, trails of hoof-like marks appeared overnight in the snow covering a total distance of some 40 to 100 miles. The footprints were so called because some people believed that they were the tracks of Satan, as they were allegedly made by a cloven hoof." (Wikipedia)