Within the depths of the underground ecosystem, two keystone species can be found.

The Glow-Back Gruntling is a small, mammalian creature, closely related to the Common Gruntling. This species developed from a population of gruntlings that ventured deep underground in search of new habitats.
These creatures differ from their relatives on the surface in a few ways: their eyes and ears have become bigger as a response to the dim conditions of the subterranean habitat, their tails became thicker and serve as fat storage for harsher times, but most notably, they have developed a series of bioluminescent organs on their backs. These organs emit a weak light which is enough for the gruntlings' eyes to see the world around them. They can, at will, turn these lights on and off, depending on the circumstances (like when they want to hide from a potential predator).
However, in order for the organs to function properly, the gruntlings need to regularly ingest a special type of moss.

The Glow Moss is an offshoot of the common Cave Moss that has developed the ability to produce its own light in the absence of Glowing Fungi.
This plant grows primarily in areas where a special type of mineral can be found: Aqua Ore. This particular type of ore has the magical property to condensate water from the air and is normally used by humanoids in the production of Water-Element Weapons and Armor. Small fragments of this mineral are also often carried by adventurers and serve as a reliable source of drinking water. In its natural state, Aqua Ore creates pools of water around it which are perfect for sustaining plant growth.
The Glow Moss, over generations of exposure to this ore, has been influenced by its ambient magical radiation and this developed the ability to produce light. When consumed over a long period of time, it can also cause whatever animal consumed it to mutate and start producing light as well. This is how the Glow-Back Gruntlings came to be.

Both the Moss and the Gruntling are important for their ecosystem; the former provides light that can be used by other plants (such as regular Cave Moss) to grow, while the latter serves to both control the plant population and provide food for other underground animals.

As a scholarly side note: the fact that Glow-Back Gruntlings can be found underground can serve as an explanation as to why their surface-dwelling relatives can be found both in Arcadia and Elysium. Assuming that the underground ecosystem runs deep enough, it is possible for it to connect both land masses, thus creating a pathway for certain creatures to travel between them. Thus far, however, the Gruntlings seem to be the only species to have successfully made this journey.