What was special about the colt the disciples brought to Jesus in Mark 11?

Answer: No one had ever sat on it

Jesus said they would find a colt tied there "on which no one has ever sat" and told them to bring it.

“and said to them, “Go into the village ahead of you, and as soon as you enter it, you will find a colt tied there, on which no one has ever sat. Untie it and bring it here.”— Mark 11:2 (BSB)