The most important reference to an ass colt is in Zechariah 9:9, predicting the coming of Israel’s king to Jerusalem riding on a colt, the foal of an ass. This Scripture is quoted by both Matthew and John in their accounts of Jesus’ triumphal entry into Jerusalem. All four gospels record this important event and the fact that Jesus rode on an ass’s colt on that occasion (Matt 21:1-11; Mark 11:1-10; Luke 19:28-40; John 12:12-19). The synoptic gospels tell how Jesus directed two of His disciples to obtain the colt. Matthew alone records that they brought both the colt and its mother (Matt 21:7). Jesus’ actions seem to be a deliberate fulfillment on His part of the Messianic prophecy. The choice of an ass rather than a horse seems to have been because the former was regarded as an animal of peace, while the latter was used for war. Furthermore, in Pal. the ass was the domestic animal most frequently used for riding. Hence Jesus’ use of it was also an evidence of His humility.
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.