Your friend's son is over-extending "Mono" to other similar animals. This is actually a normal part of a child's linguistic development.
My son also does the same thing with "dodo" for dinosaur. At first, he applied it only to big, scary dinosaurs that made large sounds—later on, it applied to any animal that had some large body and jaws, e.g. a shark. Finally, he learned to differentiate between the majority of animals (save for a few big, scary-looking animals that are still "dodo") and can name sharks, dinosaurs, and bears separately!
Yes. I don't know what the exact term for the process is, though, since I'm not knowledgeable about linguistics and childhood development beyond what I've learned from raising my son.
It's very exciting to watch and hear the progress!
My son also does the same thing with "dodo" for dinosaur. At first, he applied it only to big, scary dinosaurs that made large sounds—later on, it applied to any animal that had some large body and jaws, e.g. a shark. Finally, he learned to differentiate between the majority of animals (save for a few big, scary-looking animals that are still "dodo") and can name sharks, dinosaurs, and bears separately!