The monkey here was born at the zoo. The mother monkey would protect the child from danger, but didn’t take care of teaching it or generally taking care of it. The zoo staff took care of the monkey, but in the end Rilakkuma befriended the monkey and began acting as its friend? mother?
Here is a link to the zoos homepage showing Rilakkuma in the photos with the monkey
and the main link to thezoo home page (Japanese language only)
The address for the zoo is 市川市大町284番地 Ichikawa-shi in Chiba-ken (close to the Chuo Racetrack). The phone number is 047-338-1960.