Arock may not be the strongest dragon within the order but his knowledge and patience is well respected. His rider Rynn is the leader of the riders and though she is loud and mouthy they work perfectly as a team.
When he was young he trained under the dragon that became the great draco-lich that Ancalagon fought. So when Arock was awakened within the order it was thought to be a sign of the order’s impending doom. However in the years since no ill has befallen The Order of the Flame he soon became accepted into the group.
Even Ancalagon, who has the most reason to distrust him, accepts him and includes him in the group.