From Tolkien Gateway
Derrilyn was the name of a river in the poem Errantry by Bilbo Baggins. The name was not actually Elvish but an invention in the Elvish style.
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|
|Other:||Caves of the Forgotten · Gorbelgod|