There are rare plants and extremely rare plants, but it's unusual for there to be just one of a species.
Botanist James Medley Wood was strolling through the Ngoya Forest of Zululand in southern Africa when he stumbled upon what would end up being his most famous discovery, a single clump of a rare and ancient cycad that by 1908, shared its discoverer’s name when it was christened Encephalartos woodii, commonly referred to as Wood’s Cycad.
Wood collected three basal offsets of the plant, and later went back for two large stems. The specimens were all planted at the botanic gardens of which he was curator, the Durban Botanic Gardens in KwaZulu-Natal, where the original specimen still grows. A few of the botanical gardens that care for specimens grown from offsets of the original plant include the Kew Gardens, Longwood Gardens in Pennsylvania, Hortus Botanicus in the Netherlands, and the Botanical Garden of Naples. The lone plant that has ever been found is a male, so the only offspring are from cuttings of offsets.