In my D&D 5e game, I have a custom cosmology, and I'm currently working on a concept for an afterlife plane where any sentient creature who enters is sorted by alignment and guided to the afterlife. While in this afterlife plane, a character begins to take on a form that represents its alignment. This visually distinguishes each alignment from one another (making them easier to sort) but also provides a cool transformative aesthetic that my players have expressed interest in seeing.
In order to prevent this from being opinion-based, answers must entail specific monsters that have canonically been declared to be representative of their respective alignment, or creatures that have another narrative connection with their alignment or alignment-based plane. I will accept any creature from the history of D&D and Pathfinder as long as you can cite its close ties with its respective alignment.
I understand that there may be more than one correct answer (for example, Monster X may be the perfect NE monster in D&D 4.0 while it might be Monster Y in Pathfinder. An answer is considered correct as long as it is backed by the lore of its system.
Best Answer
In AD&D, the cosmology of D&D which had gradually developed over the course of numerous publications was codified into the Planescape setting, which focused on adventures on the planes besides the Prime Material. The architecture of the cosmology focused on the “Great Wheel,” the sixteen planes surrounding the Outlands. These seventeen planes, collectively known as the Outer Planes, were the provinces of belief: mortal belief became solid matter in the Outer Planes, so that each was made from belief.
Being literally made from belief, the Outer Planes were all about alignment. The Outlands is True Neutral, and then eight of the planes correspond to the various non-neutral alignments. The remaining eight fit in between the others, creating spaces halfway between the various extremes of alignment (so Arcadia lies between LG Celestia and LN Mechanus, and is thus Lawful and Good-ish-but-not-quite-truly-Good).
Each of the planes matching an alignment also spontaneously gave rise to its own “exemplars,” creatures formed of that plane (and sometimes the souls of mortals who ended up there in their afterlives, though the exact process is full of caveats and exceptions and beyond the scope of this answer). The exemplars didn’t merely “believe” in an alignment, they were literally that alignment incarnate.
The nine exemplar races, then, are:
Lawful Good archons from Celestia
Neutral Good guardinals from Elysium
Chaotic Good eladrin from Arborea
Lawful Neutral modrons from Mechanus
True Neutral rilmani from the Outlands
Chaotic Neutral slaadi from Limbo
Common slaad are all colored frogs, and their color denotes their variety and powers. This consistency is a notably Lawful trait imposed upon them by their sovereigns, who used the Spawning Stone to permanently fix the slaadi in this form to limit them and prevent them from becoming powerful enough to overthrow the Slaad Lords. The Slaad Lords themselves are not frog-like, and indeed can chaotically change themselves significantly. One even did a stint impersonating Primus, the top of the modron heirarchy, in an attempt to spread chaos to lawful Mechanus (or maybe just for a laugh; hard to say with slaadi), though this damaged him significantly.
Lawful Evil devils from Baator
Neutral Evil yugoloths, or daemons, of Gehenna and Hades
Gehenna is the plane between LE Baator and NE Hades, and yugoloths were originally from there, rather than Hades. The near-mythical General of Gehenna specifically “purged” the yugoloths of law and chaos with an artifact known as the Heart of Darkness. They now naturally form in both planes, making them the true exemplar of Hades, and they are neutral evil, but some caveats seemed worthwhile. They now mostly operate as notoriously unreliable mercenaries, always open to higher bidding from those they were hired to fight.
Like the baatezu and tanar’ri (below), yugoloths are not the original exemplars of neutral-evilness. The ancient baernoloths, who were the original exemplars, specifically created the yugoloths to replace them, to be the most perfect exemplars of pure evil.
Chaotic Evil demons of the Abyss
And if you’re wondering, yes, there’s a reason the Upper Planes didn’t get any sub-notes: they just never got a lot of attention. They always existed, and had some history and description, traits, stats, and so on, so they aren’t pure mysteries like the rilmani, but they also just don’t have a lot going on. Things are fairly peaceful in the heavens, as it turns out. However, while we’re on the subject, there is one thing about the Upper Planes that does warrant a mention: angels. Angels are not exemplars, and can be any good alignment (barring a fall, of course), but they are in some ways above the good exemplars. They are much, much more powerful, and represent all of Good. They are the direct hands of the good gods. Other alignments lack any similar “arch-exemplar.” So the heavens do have that going for them.
The Great Wheel cosmology was re-used throughout many editions of D&D, though significant changes were made from time to time. Third edition, and the 3.5 revision, largely kept it, except in the new Eberron campaign setting which completely eschewed it. Fourth edition nominally used... something sort of maybe like it, but mucked with most of it. Fifth edition once again is using Planescape as an official published setting.