An adult foal is a young horse that has grown to its mature stage. However, there are many other synonyms for this term that can come in handy while writing. One of the popular synonyms for adult foal is yearling, which refers to a horse between one and two years old. Another synonym is a filly or colt, which are male and female young horses, respectively. Weanling is another synonym for an adult foal, which means a horse that has been separated from its mother and is usually three months to one year old. Lastly, there's a mare or stallion, which refers to a mature female and male horse, respectively.