If you can't find nothing, try looking for something!
From the SRD:
Fiendish creatures dwell on the lower planes, the realms of evil, although they resemble beings found on the Material Plane.
"Fiendish" is an inherited template that can be added to any corporeal aberration, animal, dragon, fey, giant, humanoid, magical beast, monstrous humanoid, ooze, plant, or vermin of nongood alignment (referred to hereafter as the base creature).
Celestial creatures dwell on the upper planes, the realms of good, although they resemble beings found on the Material Plane.
"Celestial" is an inherited template that can be added to any corporeal aberration, animal, dragon, fey, giant, humanoid, magical beast, monstrous humanoid, plant, or vermin of good or neutral alignment (referred to hereafter as the base creature).
So, theoretically, a neutral base creature could acquire both templates. It would have to born that way, likely the offspring of a celestial and a fiendish parent (probably of the same base creature type). It would involve some kind of planar travel, or at least some summoning, but that shouldn't be hard for a high-level wizardly breeder of exotic creatures.
Oh if your wondering what I am cooking up I read a thread on here about elder brains and making one half dragon or half demon to make them fiendish or celestial. So why not both and see what happens. We have been playing for years and never one time have i made a monster befor and they keep asking me why. I just tell them you dont want me to but they will not leave me alone about it. www.giantitp.com/forums/showthread.php?303094-What-the-heck-are-Elder-Brains
