A filly is a young female horse. Horse people usually mean a girl horse up to about four years old; after that she is a mare. A young male horse is a colt, and a baby of either sex is a foal — so filly is more precise than "baby horse". You will hear it at stables, horse races, and on farms. Older English sometimes used filly for a lively young woman, the way some people once said "a young mare"; that use can feel dated or a bit teasing now, and it is kinder to keep the word for horses unless you are reading an old book. It is not a misspelling of fully.