Zechariah 9:9
King James Version (KJV)
9 Rejoice greatly, O daughter of Zion; shout, O daughter of Jerusalem: behold, thy King cometh unto thee: he is just, and having salvation; lowly, and riding upon an ass, and upon a colt the foal of an ass.
in hebrew poetry, the second line repeats a concept in the first line in different words known as parallelism. "a colt the foal of an ass" repeats the concept of "riding upon an ass" in the first line. this is not talking about 2 donkeys, it is merely repeating a concept in different words. Mark understands this.
Mark 11:7
King James Version (KJV)
7 And they brought the colt to Jesus, and cast their garments on him; and he sat upon him.
apparently matthew doesn't understand hebrew poetry's parallelism.
Matthew 21:5-7
King James Version (KJV)
5 Tell ye the daughter of Sion, Behold, thy King cometh unto thee, meek, and sitting upon an ass, and a colt the foal of an ass.
6 And the disciples went, and did as Jesus commanded them,
7 And brought the ass, and the colt, and put on them their clothes, and they set him thereon.