Graydon Hoare created Rust as a personal project while working at Mozilla Research in 2006 - https://en.wikipedia.org/wiki/Rust_(programming_language)

Not many devs create the core of what is arguably the best programming language available today as a personal project. However, it's true that what it has become today is now the work of many.

A good deal of programming languages started out this way. A few that come to mind: C, C++, and Python. Javascript.