Contradiction with Matthew 21:7
This verse describes Jesus entering Jerusalem on two animals - a donkey and a colt - whereas John 12:14 mentions only a young donkey.
Matthew 21:7: And brought the ass, and the colt, and put on them their clothes, and they set [him] thereon.
Contradiction with Mark 11:7
This verse describes Jesus riding on a colt, which could imply it was not specifically a young donkey as implied in John 12:14.
Mark 11:7: And they brought the colt to Jesus, and cast their garments on him; and he sat upon him.
Contradiction with Luke 19:35
This verse also mentions Jesus riding on a colt without specifying that it was young, as stressed in John 12:14.
Luke 19:35: And they brought him to Jesus: and they cast their garments upon the colt, and they set Jesus thereon.