Abaris natives were less than two feet tall, green, and mushroom-shaped. Similar to a mushroom, their heads consisted of a flattened cap and gills, though they also had a wide mouth and two large eyes with narrow pupils. The stems of the natives were more humanoid, however, containing ridges, arms and legs rather than mycelium. They also had a cape-like covering.
They might reproduce through similar processes as mushrooms, by releasing spores through their gills.
Their fungal-based physiology lent itself to a serene lifestyle. They were peaceful, artistic, and highly communal, always seen in groups.
They demonstrated an ability to read human thoughts at close range. Historical evidence also suggested a long-range capability for telepathic reception. They did not reveal an ability to transmit their thoughts, at least with species other than their own.
More than 5,000 years ago, Abaris natives invented hieroglyphics writing and dabbled in astronomy. As artisans, they became adept in carving. Through precognition they knew that their planet would be destroyed and that their species' future depended on Humans from the Federation. They reached out telepathically to find out what they could of Earth, learning about the Sumerians.
5,000 years ago, the Abaris natives built a vast stone pyramid with sculptures in the walls celebrating not only their own species but many other mushroom species, illustrating their evolution or history. They carved a floor tile with Rosetta Stone-like inscriptions in their own language and the language of the Sumerians. The natives also created a 10-foot tall sculpture of a Federation starship.
Within the pyramid, they cultivated many giant-sized varieties of non-sentient mushrooms. Whether the pyramid was constructed mostly underground to begin with, or became buried over the centuries, at some point it and the rest of its civilization seemed to have vanished. Only the top nine meters of their pyramid remained above the surface.
Circa 2270, archaeologists Paul Hoff and Elsa Hoff established a small outpost to research the lost civilization. They eventually discovered surviving natives, immersing themselves in their culture and language.
In 2273, the USS Enterprise rescued the two archaeologists and 2,000 surviving natives. They witnessed the planet's annihilation by a rogue planetoid from a safe distance, then were taken to Starbase 11. Leonard McCoy recommended that the Hoffs be supervised by a psychochemist as they assisted in relocation efforts, to mitigate side effects from continued proximity to the natives. (TOS comic: "Aberration on Abaris")