Can a good creature be fiendish and can an evil creature be celestial?

[Shemeska]

seems they skipped out on the redeemed fiend, however.

They exist, both in multiple editions of D&D and Pathfinder.

 

[the Jester]

Seriously: if you have to ask, you don't understand what the rules of the game are for. They're there as a structural concept that is meant to help you tell a good and interesting story. Rules are supposed to be facilitators of good stories, not to cramp your style.

You make this sound absolute. It's true for some playstyles, but in others, the whole idea of "telling a story" is anathema. For some folks, the rules are there to simulate the world, and the RAW are the rules of reality for that campaign. I think it does a disservice to those who play that way to imply that they are doing it wrong.

Specific to the OP, remember that even a creature that is listed as "always evil" does have the rare exception; it's just that they are damned rare.

 

[nijineko]

They exist, both in multiple editions of D&D and Pathfinder.

not say they can't exist by rules, just that in the story setting material, they don't seem to have one. (that i can recall).

 

[Shemeska]

not say they can't exist by rules, just that in the story setting material, they don't seem to have one. (that i can recall).

Here are a few that have showed up in D&D.
Fall-From-Grace - Risen succubus (PS:Torment and its novelization)
Felthis ap Jerran - Risen ultroloth (Dragon 351)*
K'rand Vahlix - Risen hamatula (Faces of Evil: The Fiends)
Nalura - Risen erinyes (Planes of Conflict)

There have been a lot more in Pathfinder as well.
Arueshalae - risen succubus servitor of Desna (prominant NPC in the Wrath of the Righteous adventure path)
There are others, just none others I can think of by name off the top of my head.

*blame me here for this one

 

[nijineko]

Here are a few that have showed up in D&D.
Fall-From-Grace - Risen succubus (PS:Torment and its novelization)
Felthis ap Jerran - Risen ultroloth (Dragon 351)*
K'rand Vahlix - Risen hamatula (Faces of Evil: The Fiends)
Nalura - Risen erinyes (Planes of Conflict)

There have been a lot more in Pathfinder as well.
Arueshalae - risen succubus servitor of Desna (prominant NPC in the Wrath of the Righteous adventure path)
There are others, just none others I can think of by name off the top of my head.

*blame me here for this one

that would explain it, i have limited play exposure to planescape (despite making frequent use of the official 3.5 planescape material), and I tend to skip evil-outsider articles in dragon or evil-outsider themed source books, not having much use for them, due to the quirks of my campaign settings.

and for the other, i refuse to play pathfinder, due to a lack of psionic support in core material. personal quirk.

congrats on the blame, btw. ^^ and thank you for the information. =D