One of the last northern white rhinos known to exist has passed away from what are believed to be natural causes.

Suni was the first white rhino to be born in captivity 34 years ago at the Dvůr Králové Zoo. The beloved rhino was one of four that was transported to the Ol Pejeta Conservancy in 2009, where he died on Oct. 17, 2014.

"It is with great sadness that we announce the death of one of our northern white rhinos, Suni," the park said in a statement.

The rangers found the beloved rhino in the morning in his Boma. He was not believed to have been a victim of poaching, but the cause of his unexpected death has not yet been determined, and the The Kenya Wildlife Service vets are planning to conduct a post-mortem examination.

In 2006, Suni's father, Saút, died in the Dvur Kralove Zoo from natural causes at the same age.

There are now believed to be only six white rhinos left on Earth; Suni was one of the last two breeding males and his death has put the species in extreme danger of extinction.

As of the late 1960s, over 2,000 northern white rhinos still walked the Earth, but by 1984 there were only 15 remaining, and poaching has caused those numbers to continue to decline, the World Wildlife Fund (WWF) reported.

The northern white rhino originally lived in southern Chad, the Central African Republic, southwestern Sudan, northern Democratic Republic of the Congo (DRC), and northwestern Uganda before being almost completely wiped out by poachers.

In the future, the team hopes to turn the trend around.

"We will continue to do what we can to work with the remaining three animals on Ol Pejeta," the reserve said, "in the hope that our efforts will one day result in the successful birth of a northern white rhino calf."