FOAL (עַ֫יִר, H6555, meaning donkey, stallion, or colt; but more often בֶּן meaning son of—). The word “foal” refers to the young of a horse, donkey, or zebra. In the Bible it is used in relationship to the young of a donkey. It is often interchanged in some tr. with the word “colt” (Gen 32:15; 49:11).
The most common way in the OT and NT to designate a “foal” or a “colt” is to use the phrase “the son of a donkey” (Zech 9:9 and Matt 21:5).
International Standard Bible Encyclopedia (1915)
The English words "colt" and "foal" are used in the Bible of the ass everywhere except in Ge 32:15, where the word "colt" is used of the camel in the list of animals destined by Jacob as presents for Esau. In most cases `ayir (compare Arabic `air, "ass") means "ass’s colt," but it may be joined with ben, "son," as in Zec 9:9, where we have: `al-chamor we`al-`ayir ben-’athonoth, literally, "on an epi onon kai epi polon huion hupozugiou", "upon an ass, and upon a colt ass, and on an ass’s colt, the son of the she-asses"; compare Mt 21:5 epi onon kai epi polon huion hupozugiou, "upon an ass, and upon a colt the foal of an ass." In Joh 12:15 we have epi polon onou, and in the previous verse the diminutive, onarion. The commonest New Testament word for "colt" is polos, akin to which is German Fohle and English "foal" and "filly." The Latin pullus signifies either "foal" or "chicken," and in the latter sense gives rise to French poulet and English "pullet."
In view of the fact that horses are but little mehtioned in the Bible, and that only in connection with royal equipages and armies, it is not surprising that "colt" does not occur in its ordinary English sense.