Jhn 12:12
On the next day much people that were come to the feast, when they heard that Jesus was coming to Jerusalem,
Jhn 12:13
Took branches of palm trees, and went forth to meet him, and cried, Hosanna: Blessed is the King of Israel that cometh in the name of the Lord.
Jhn 12:14
And Jesus, when he had found a young ***, sat thereon; as it is written,
Jhn 12:15
Fear not, daughter of Sion: behold, thy King cometh, sitting on an ***'s colt.
Mat 21:2
Saying unto them, Go into the village over against you, and straightway ye shall find an *** tied, and a colt with her: loose them, and bring them unto me.
Mat 21:3
And if any man say ought unto you, ye shall say, The Lord hath need of them; and straightway he will send them.
Mat 21:4
All this was done, that it might be fulfilled which was spoken by the prophet, saying,
Mat 21:5
Tell ye the daughter of Sion, Behold, thy King cometh unto thee, meek, and sitting upon an ***, and a colt the foal of an ***.
Mat 21:6
And the disciples went, and did as Jesus commanded them,
Mat 21:7
And brought the ***, and the colt, and put on them their clothes, and they set him thereon.
John focuses on the young *** Christ sat on, Matthew mentions the mother *** came along too.
The Gospels do this all the time, one Gospel adding detail to the other's story.
The error that I pointed out did not concern the number of animals that were riden.
I pointed out that at least one gospel misquotes Jesus in His Words to the two disciples he sent to find and bring the animal(s).
So it is not a case of adding details. It is a case of contradiction.