You wouldn't happen to know why Jonathan Harris was listed as "special guest star" on the show's credits (for its entire run) would you? It seemed like almost every episode pivoted around his character.
There is a clip of Jonathan Harris telling that story on YouTube. Basically, it is how Irwin Allen convinced him to be in the series. He wanted to get top billing in the credits, but that was already promised to the other actors (as part of their contract). So the compromise was to make him a “Special Guest Star” in the credits.
Not sure it was supposed to be a permanent arrangement. His character was supposed to get killed off after a few episodes, but got popular enough that those plans were changed. So, he just kept being a Special Guest Star.
In Hollywood, vanity is serious biz.