The Terries and the Fermies are two kinds of creatures who live underground. They first appear in Land Beneath the Ground! by Carl Barks. They also appeared in the DuckTales episode, "Earth Quack", albeit the DuckTales version merges both species into a single race called Terra-Firmians.
Terries and Fermies tell each other apart by the ties they wear on ribbons around their necks — the Terries wear bow ties and the Fermies wear four-in-hand knots. They locomote by folding their head and arms into their body so they become a perfect sphere and then they roll from place to place like a bowling ball. They are colored various bright colors.
They have periodic contests on an irregular basis to see who can cause the biggest earthquakes. Terries cause earthquakes by rolling in massive groups of thousands against the giant pillars that support the land on the surface of the earth.
Fermies cause earthquakes by gathering together in massive numbers and lifting up pieces of the Earth's crust. A rolling Terrie is strong enough to defeat Donald in a headbutting contest. Whichever group causes the biggest earthquake wins the contest and gets the prize, a piece of Ancient Greek pottery that fell down a crevice in Ancient Roman times (later replaced by Scrooge McDuck's hat at the end of the story).
The Terries and Fermies make a cameo in The Universal Solvent by Don Rosa, and in Creature Comforts by William Van Horn, Donald and Huey, Dewey and Louie must help a Fermie to find his way back home. In Scrooge's Last Adventure (Uncle Scrooge #417-420), Scrooge and his nephews return to the Terries and the Fermies.
In 2017's DuckTales, Terra-firmians are figures of urban legend lurking the Duckburg subway tunnels and sightings were reported in the newspapers. They appear properly in the episode "Terror of the Terra-firmians!". According to the urban legends recorded by Webby, the Terra-firmians were two tribes known as Terries and Fermies that ended up becoming united by a single king and their friendly competitions created earthquakes. After his death however, a civil war broke out and the king's tribe were forced out of power. The Terra-Firmie Prince began leading a resistance to reclaim the throne and this war has been the primary cause of quakes since. However, this appears to be merely a story, as the episode's Terra-firmians have an identical story about the "Duck-billed Surface Dwellers". Unlike prior versions, they also appeared to not wear any form of clothing.