First introduced in Larry Nivens Known Space novel Protector, was the fate of the Brennan Monster, Earth's first Human Protector, ever revealed in the Ringworld book series? If so, which book tells the tale?
Do you mean Brennan or Truesdale?
Brennan's fate is described in Protector:
Brennan and Truesdale travel to the planet Home to distract the Pak fleet from Earth, on the way Truesdale realizes Brennan's plan is to infect everyone on Home with Tree-of-Life, so he kills Brennan to try to prevent that from happening.
What happens to Truesdale...
and the rest of the Protectors after everyone on Home is either killed or converted to a Protector by the Tree-of-Life infection and they get ready to fight off the Pak fleet...
is not covered in any of the Ringworld novels.
(This could probably just be a comment, and given how old Protector is, probably doesn't need the spoiler markup, but just in case...)