"Staff of the Save the Rhino Trust found a female black rhino killed and its horns had been removed," an official of Namibia's ministry of environment and tourism told AFP on Thursday.
"One man has been already arrested and the two rhino horns were found and confiscated."
Namibia's Save the Rhino Trust discovered the dead rhino on Sunday in the remote Damaraland area of Namibia's northwestern Kunene Region, some 600km northwest of the capital Windhoek.
The female had been with a young calf, which could "unfortunately not be rescued, it was too weak and died", the official said.
Rhino calves stay near their mother even if she is killed.
The deaths are a rare blip in otherwise successful protection of the rhino in Namibia, which sees years go by without an incident of rhino poaching being reported.
During the 1980s, poaching was rampant in northwestern Namibia, but after independence in 1990, it was brought under control and the black rhino population increased.
Namibia is now home to more than 2 000 rhino and the death raises concern that industrial-scale poaching that has hit South Africa this year may be spreading.
At least 633 rhino have been killed in South Africa this year, fuelled by demand from Vietnam and elsewhere in Asia, were horn is variously believed to be an aphrodisiac, an anti-carcinogenic and an amulet.