Conceptual Development: In Silmarillion drafts from the 1930s, this river first appeared as Eldor, later revised to Eglor (SM/227). In The Etymologies, the name Eglor was designated Ilkorin and translated “Elf-river” (Ety/ELED), and was given as the basis for the name Ilk. Eglorest (Ety/RIS²), the earlier form of S. Eglarest. In the map revisions for the Silmarillion from the 1950s-60s, Eglor was replaced by S. Eglahir and later still by Nenning (WJ/187), which was the form used in the later Silmarillion narratives (WJ/117).
Note that the name Nenning first appeared in Lord of the Rings drafts from the 1940s as a rejected name for the rivers N. Crandir “Redway”, which itself was an earlier name for the river Celebrant (RS/433).