A "mare foal" refers to a baby horse that is born to a female horse, or mare. There are several different synonyms that can be used to describe this young animal. One such term is "filly," which is used to describe a female horse that is under the age of four years old. Another similar term is "colt," which refers to a male horse that is also under four years in age. Some other synonyms for "mare foal" include "young horse," "yearling," and "weanling." Each of these terms can be used to describe a different stage of development for a young horse, depending on factors such as age and gender.