The Old Scientist is an aged, wise man who has been studying the Fire-Eater for over forty years. He knows a lot about this bird, so it is safe to assume that he has a vast knowledge of biology in general and ornithology in particular.
At some point during his researches, he decided to travel to the volcanic Mount Kiwi, the home of the Fire-Eater, but all that he could find there was the last egg of this seemingly extinct bird species. He then decided to stay in the island and took care of the egg for many years, hoping that it would hatch some day.
The years passed and the scientist thought that the egg would never hatch until the day that Goku and Chi-Chi came to Mount Kiwi searching for the Fire-Eater's feathers, which they believed would be necessary to create the Bansho Fan. Emperor Pilaf also came here searching for the egg to steal it and later sell it, but Goku stopped his evil plans once again by destroying Pilaf's drill vehicle. When the vehicle exploded, the only remaining adult Fire-Eater appeared and, much to the surprise of the old scientist, the egg finally hatched.
The adult Fire-Eater and his cub then disappeared in the sky and when Goku and Chi-Chi tried to catch some of the adult's feathers, they just faded away. The old scientist then told them that the Fire-Eater's feathers could never be used to make the Bansho Fan, but, luckily for the young couple, he knew where the real fan was: at Mount Frappe, a snowy mountain located in the far north.
Goku and Chi-Chi then said goodbye and left Mount Kiwi, leaving the old scientist alone there. It is unknown what happened to him after that and if he also left the island to return to his home since he never appeared again.