Filly
来自Big Physics
late Middle English: from Old Norse fylja, of Germanic origin; related to foal.
wiktionary
From Old Norse fylja (whence Danish føl), related to foal.
etymonline
filly (n.)
c. 1400, filli, fyly, "a young mare, female colt or foal," possibly from Old Norse fylja, fem. of foli "foal," from Proto-Germanic *fuljo, from PIE *pulo- "young of an animal," suffixed form of root *pau- (1) "few, little." Slang sense of "lively young girl" is from 1610s.