To find out how to bring Mosskit/Moss Shadow back, you have to talk to Moonheart after you become a warrior. She'll tell you about Mosskit/Moss Shadow and a red X will appear at the FireClan border. I don't want to make it too easy, but I will tell you that you have to place a very specific item on the X. You can find certain items in areas, but herbs are not any of the items. Then you have to wait until night to go back to the FireClan shore border. If you placed the correct item, Mosskit/Moss Shadow should appear