Rhino horn is used in traditional Asian medicine to fight malaria, epilepsy, fevers, and other ailments. In Yemen the horns are in demand as carved handles on traditional daggers.
IUCN says poachers likely gave the final push that sent the subspecies into oblivion.
Also last week the environmental group reported grim news for Africa's northern white rhino.
Ground and aerial surveys of the animal's last known holdout, in Garamba National Park in the Democratic Republic of the Congo (map of the Congo), identified just four animals.
"Efforts to locate further animals continue," said Martin Brooks of ICUN's African Rhino Specialist Group.
"But we must now face the possibility that the subspecies may not recover to a viable level."
The bleak findings contrast with the improving outlook for rhinos elsewhere on the continent.
In southern Africa the southern white rhino has turned into a striking conservation success story, rebounding from fewer than 50 animals a century ago to over 14,000 today.
And the three other black rhino subspecies have increased in number, particularly in South Africa, Namibia, Kenya, and Zimbabwe.
The continent's total black rhino population gained from a record low of 2,410 in 1995 to 3,725 animals today.
The tally grew 3.5 percent in the past two years alone, according to IUCN.
But those figures still pale in comparison to the rhino's historic numbers (related wallpaper: black rhino in 1909).
According to some estimates, there were 14,000 black rhinos as recently as 1980 and more than 100,000 in 1960.
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