In D&D lore, each alignment has a representative creature that embodies that alignment. They are:
- Lawful Good Archons (Angels)
- Neutral Good Guardinals
- Chaotic Good Eladrin
- Lawful Neutral Modrons
- Chaotic Neutral Slaadi
- Lawful Evil Devils
- Neutral Evil Yugoloths
- Chaotic Evil Demons
The opposite of Lawful Neutral is Chaotic Neutral, so Slaadi are the opposite of Modrons.
This, but with a few clarifications.
The most developed form of this all got established during 2e, in which there ended up being 9 Exemplar races. Exemplars are "afterlife beings", formed from the souls of dead mortals.* Each type of Exemplar has various "sub-species".
Archons aren't really analagous to angels--angels(also known as Aasimon) are something different than the Exemplar races. Archons, Guardinals, and Eladrin, as good Exemplars, are conceptually similar to angels though. The Archon subtypes vary a lot, from balls of light, to animal headed people, to something that looks much like an angel.
These Eladrin are completely different than the 5e eladrin, which is a continuation of the 4e eladrin, which was an infuriating case of identity and name appropriation. As explained, original Eladrin are afterlife beings, usually quite powerful (like fiends), not a simple PC level character option. They have a fey-ish aesthetic, but each type is unique, including pixieish types, to firey humanoids, and others. They are not related to the actual fey lords like Titania and Oberon.
Guardinals are anthropomorphic animal people. The most powerful subtype (the Leonal) is kind of like a bipedal Aslan from Narnia, which gives you a bit of their theme.
The ninth type, Rilmani, is the True Neutral Exemplar. They live near the Spire at the center of the Outlands, and the types look like hairless humanoids made of different types of metals.
Devils call themselves Baatezu, Demons call themselves Tanar'ri, and mortals without extensive planar knowledge sometimes call Yugoloths "Daemons".
Slaadi look the way they do due to the power of the Spawning Stone. 5e decided it was Primus who made that happen, but I'm pretty sure that's a new take--kinda like how 5e wants "it's all about Asmodeus" to be the fiend theme and reimagines the origin of the Yugoloths accordingly.
If you can find a download of the authorized 3e Planescape campaign setting, you can read more on this. They also give a description of Slaad (I never remember which form is plural, and I think it may have changed with editions) behavior that puts them into a better CN perspective, rather than the CE way they tend to be presented (though, to be fair, it has been explained that the CE found in the more powerful varieties is a corruption, not their natural state). I highly recommend grabbing the pdfs and considering that take on Slaad(i).
In fact, I highly recommend grabbing that 3e Planescape setting (it's free) in general, as the fluff sections gather together the core lore from 2e. If you want more details the 2e materials are available for purchase on DMs Guild. It provides a really rich take on what it is like on the Outer Planes. While mortals have an understandably simplistic view of the afterlife (demons are different from devils, right?) there is a whole complex multiverse of various beings and the strange dimensions they live in for those who want to make more than a brief visit (and even societies of mortal humans and others who have been living there for ages!)
*Not all Exemplars are formed from the souls of the dead. It depends on a variety of factors, and some specifics can be unclear or variable. Personally, I just say all 9 types are, even if some of them get there differently.