They are Zack's three siblings. They was born to the pack leader with Zack Before they were even named, the curious puppies ran to explore and were found by a human. The human took Zack and his siblings inside his knapsack and ran. Hidaka Reiji saw this and chased the man with his minions. The scared human threw the knapsack into the river. Hidaka Reiji rescued the puppies and thus peace was created between the two packs.