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.