What did Pete mean when he said: “When he was made, the mold was broke.”?

Pete means that Buck was one of a kind. Buck was a dog the likes that they had never seen before. He was strong beyond comprehension, loyal, smart and a natural leader of dogs.

Never was there such a dog," said John Thornton one day, as the partners watched Buck marching out of camp.

"When he was made, the mold was broke," said Pete.