PORTLAND, Ore. -- A rare black rhino died Monday night during a trip from Kansas City to Portland.

Zoo officials said the rhino became agitated during transport early Monday morning, causing the truck to sway back and forth.

Zoo staff said they stopped the truck about 20 miles outside of Phoenix, Ariz., to check on the rhino and discovered the female in distress. They turned the truck around and headed for the Phoenix Zoo.

Gilbert Gomez, an assistant curator for the Oregon Zoo, said the Phoenix Zoo staff used a crane to lift the rhino's crate from the truck. The rhino was then unloaded into the zoo's elephant barn.

An Oregon Zoo veterinarian headed to Phoenix to help care for the sick animal. The rhino died Monday at 9:30 p.m.

A necropsy is under way to determine the rhino's cause of death.

The rhino, named Kipenzi (Swahili for "precious one"), was born at the Kansas City Zoo in 2000. Zookeepers planned for the rare 8-year-old female rhino to breed with a 21-year-old male at the Oregon Zoo.

“Black rhinos are among the world's most endangered mammals,” said Mike Keele, the Oregon Zoo's deputy director. “This is a devastating loss for us and for accredited zoos across the nation.”

Only 3,700 black rhinos remain in the world, according to the Oregon Zoo. There were 65,000 black rhinos living in 1970, officials said.