From Longman Dictionary of Contemporary Englishfillyfil‧ly /ˈfɪli/ noun (plural fillies) [countable] HBADSHa young female horse → colt, foal
Examples from the Corpus
filly• It had been filly embraced as the true faith.• Eight fillies were entered for the first local 2-year-old race of 1996 on Wednesday.• The flying filly had trainer Jack Berry purring after a stunning success at Beverley last month.• I want Marking Time and that three-year-old filly of yours to run at Kempton on Saturday.• Something was clearly wrong with the Mr Prospector filly, and maybe not just the faster ground.• She loved this filly and would have been so proud.Origin filly (1400-1500) Old Norse fylja 