Auckland Zoo's male rhinoceros was euthanised on Wednesday after a sudden untreatable illness. 

Last weekend, 20-year-old Inkosi suddenly stopped eating and became very lethargic, Auckland Zoo said in a statement. 

Inkosi loved to wallow in the mud. Photo / Auckland Zoo 

The rhino was administered pain relief and antibiotics but after vets did a full examination of the animal it was found Inkosi had signs of kidney failure. 

The zoo said with subsequent treatments failing and additional tests showing Inkosi's condition had deteriorated the decision was made to euthanise the animal late Wednesday. 

The initial findings of a post-mortem examination have confirmed severe kidney failure, along with abnormal changes in the rhino's intestinal track, although it will be several weeks before the cause of this can be determined. 

Inkosi came to Auckland Zoo from Hamilton Zoo in 2007 and the specialist team which cares for the zoo's rhino herd said he was an exceptionally lovely rhino who will be greatly missed. 

"He was very gentle and friendly, loved a mud wallow, especially in the rain, always enjoyed a good scratch and a food treat of lucerne, and could quite often be caught stealing Coprosma – another food favourite – from the nyala and waterbuck!" 

Auckland Zoo's remaining rhino herd includes Inkosi's dad, Zambezi (32), adult female Jamila (10) and Zambezi and Jamila's young rhino calf, female Nyah (almost two years). Jamila is now also pregnant again to Zambezi, and is due to give birth this spring.