Doriathrin

Doriathrin or the language of Doriath, was a form of Sindarin used during the First Age which preserved many archaic forms.

One difference from Sindarin was the plural form. Unlike Sindarin, the plural suffix is -in, as in orthin the plural of orth).

The state of Doriathrin, as a language or dialect, relating to Sindarin proper is not fully known. In the annals the names Dolmed, Menegroth, Mablung, Nauglamîr, Dagnir Glaurunga and Dior refer to names of the speech of Doriath. Daeron is proper Sindarin form of the Doriathrin name Dairon.

In general, Doriathrin seems not to have been derived from Old Sindarin. For example OS *botto (from PQ *mbottô) brings proper Sindarin both; but the same word in Doriathrin is moth so it can't be derived from *botto