What is the term for a young female horse?

The term for a young female horse is "filly." This classification is specific to equine terminology, where a filly refers to a female horse that is typically less than four years old. After this age, she would usually be referred to as a mare. This distinction is important in the study of animal science as it helps to understand the developmental stages and gender classifications within the horse population.

The other terms provided refer to different animals: "doeling" pertains to a young female goat, which highlights the terminology diversity in different species. A "castrated male goat" is described by a different term altogether, which does not relate to a female horse. Lastly, "ewe lamb" is a term for a young female sheep, further emphasizing the importance of knowing specific terminology within various animal species. Thus, recognizing the correct term for a young female horse helps in mastering equine knowledge within animal husbandry and science.