Gito is shown in newly-released images swinging happily and confidently on a climbing frame along with other orphaned baby orangutans undergoing rehabilitation.
His healthy appearance is in stark contrast to the lifeless figure he cut when rescuers from International Animal Rescue (IAR) found him some three months ago.
Lying corpse-like with his arms folded across his chest, his grey flaking skin and lack of hair made him look “almost mummified” in his urine-soaked box.
He was found in the village of Hamlet Giet in Simpang Hulu district, 105 miles (169km) from the orangutan rehabilitation base in West Borneo.
Dehydrated and malnourished after being fed entirely on condensed milk, Gito was taken to the IAR clinic by motorbike in an arduous nine-hour journey.
During a medical check, Gito was feverish, with stiff hands and feet.
He was unable to sit up on his own, suffering from diarrhoea and from sarcoptic mange — a highly contagious skin disease.
But in quarantine, he has developed enough strength in his arms and legs to climb and swing like other orangutans his age, a spokesman for IAR said.
IAR chief executive Alan Knight said they had taken in an increasing number of baby orangutans amid devastating forest fires in the region.
Mr Knight said: “At least Gito and his friends are in safe hands now and progressing well.
“Once they are bigger, stronger and more confident they will move on to baby school, the next stage of their rehabilitation and a step closer to their eventual release back into the wild.”