Having put out at least one solo album prior to joining or forming a band they were perhaps better known for. Bands named after or including the artist’s name don’t count (e.g. if they initially recorded solo and then added members but kept recording under their name, or added “and the…” to their name, or like The Jimi Hendrix Experience, etc.)
Only know David Coverdale before he joined Whitesnake, and I only learned that recently. He released two albums before forming the band, the first being called White Snake.
Tempted to say if they had a solo stage name or pseudonym it wouldn’t count but that would exclude a lot of otherwise perfectly fair submissions. Like rappers and electronic musicians. So it’s fair.
Oh Adrianne Lenker has some teen country pop, then some very good proto big thief stuff (but that’s a split with Buck Meek so might not count). Some lovely songs on that record too