From Tolkien Gateway

Derrilyn was the name of a river in the poem Errantry by Bilbo Baggins.[1] The name was not actually Elvish but an invention in the Elvish style.[2]

It is unknown if Derrilyn referred to an actual river in Middle-earth, or if it was just an invention by the poet.


Legendary locations in Arda
Islands:  Isle of Meneltarma
Mountains:  Astulat · Mountains of the Moon · Merlock Mountains
 Regions:  Aerie · Belmarie · Fantasie · Last Desert · Sindanórie · Thellamie · Tode
Rivers:  Derrilyn
Other:  Caves of the Forgotten · Gorbelgod