User blog comment:SayuriDarling/Full first chapter of The Mark of Athena/@comment-70.45.4.195-20120602023916/@comment-173.66.68.179-20120602165914

I think you mean Percy's knee. And Jason is riding Tempest, the storm spirit he rode into battle with in The Lost Hero. Arion is Hazel's horse and only she can ride him because she is the best female warrior.