The words ᴱN. bregil “mare” appeared in Early Noldorin Word-lists along with a variant brois, both feminine forms of ᴱN. brog “horse” (PE13/139). In the Gnomish Lexicon of the 1910s, there was a different list of “mare” words: G. bros, bross and broch, though still based on G. brog; the earliest version of the entry had only the form bross (GL/24).

Neo-Sindarin: Since -il continues to appear in later Sindarin as a feminine suffix, I would adapt this word as ᴺS. regil “mare” based on the later root √ROK used for “horse” words from the 1930s and forward; compare for example [N.] brennil “lady” versus [N.] brannon “lord”, both based on brann.