[RPG] Are there any plant species that grow only in the Feywild

D&D Sources

I think Weckar E. is basically correct in that this confusion stems from the fact that, like many elements of D&D, the fey courts are cobbled together from a mishmash of not-terribly-consistent folklore. There seem to be two competing views:

1. The two courts are basically mirror images of each other - the Seelie project an image of light and the Unseelie one of darkness, but under the surface they are pretty similar, with members of both courts being fickle and beauty-obsessed.
2. A view with more overtones of class warfare, where the Seelie are beautiful and exclusionary while the Unseelie are nightmarish and inclusive.

It sounds like more sources tend toward the first view, but I think there's enough material provided for you to choose whichever one you feel is right for your game.

Other Sources

Other depictions of the two courts that I'm familiar with tend more toward the first view. Often fey from both courts are equally dangerous in their own ways - they merely express their unpredictable personalities and the power of nature differently.

The Dresden Files novels, for example, play up the Summer and Winter connections. The Seelie are bright, colorful, and generally pleasant enough on the surface, but they can also be quick to anger and terrible in their power, like a summer's day giving way to a powerful thunderstorm. Alternatively, they might simply get carried away or be otherwise heedless of the consequences of their actions, turning from pleasant warmth to a dangerous heat wave.

The Unseelie are cold and vicious, but aren't entirely without compassion. After all, winter still has its mild days; it is not an unrelenting blizzard. They still experience some form of love, or at least emotional attachment, and while they might see mortals as playthings most of the time, they may develop a certain admiration for mortals who prove themselves capable of keeping up. Even then, mortals must remain on their guard - it's not uncommon for the Unseelie to have a sadomasochistic streak, and their version of kindness might not seem so pleasant to others.

In Wizards Presents: Worlds and Monsters, Matthew Sernett and James Wyatt explain how and why they redefined eladrin for 4th Edition. At the time when this book was written, 4th Edition had yet to be released (and, apparently, the plural of "eladrin" was "eladrins"):

In 3rd Edition D&D, eladrins were a kind of celestial being designed by dividing up the alignment “pie” among divine servants. The warrior archons were lawful good, the animal-headed guardinals were neutral good, and eladrins got the chaotic good slice of the pie.

We knew that we no longer wanted to design monsters according to that rubric. Good-aligned creatures can be useful, but D&D simply doesn’t need that many of them. [...] Also, since our cosmology no longer hinged upon the alignment-based concept of the Great Wheel, we didn’t need to create planar inhabitants for every conceivable alignment combination.

But even though the Great Wheel, its many Outer Planes, and their various races were scrapped for 4th Edition's cosmology, the eladrin were preserved (or retconned)—now as the "banner race" for the Feywild, rather than Arborea—and as exclusively fey creatures, rather than celestials:

As we discussed what to do with [eladrin], we noted their generally fey appearance, and this led to a natural association with the Feywild. [...] Eladrins were already powerful magical beings in previous editions of the game. Now they have a very similar role, but as mysterious lords and ladies of the Feywild.

The advent of 5th Edition saw another cosmological shakeup: The Feywild, a popular element of 4th Edition, was preserved, but the Outer Planes, including Arborea, returned to the default cosmology. However, the eladrin subrace as presented in the 5E DMG, Unearthed Arcana, and finally Mordenkainen's Tome of Foes are all "creatures of the Feywild" derived from the 4th edition version.

So there are two versions of eladrin, and the difference is only a matter of what edition you're looking at: Before 4th Edition, eladrin were celestials from Arborea; beginning with 4th Edition, they're fey creatures from the Feywild.