A number of fox cubs were killed and another burnt in a deliberate grass fire in the Cynon Valley.
Dog walker Gavin Thomas found the scorched cub hidden in hedges close to the bodies of other cubs.
RSPCA Insp Gemma Cooper said it was "very lucky" and was receiving treatment.
South Wales Fire and Rescue Service said the fire, between Mountain Ash and Ynysybwl, had been started deliberately.
It is hoped the cub, which was initially mistaken for a puppy, will be released back into the wild later this year.