Zhou Chun commanded the "Golden Rainbow Sword" to split open the chest and belly of the Serpent Lizard Beast, then extracted a black Demon Core from inside.
This Serpent Lizard Beast was considered a water attribute Magical Beast, which didn't suit the attributes of any of Zhou Chun's Spirit Pets. He could either find someone to refine it into an Elixir or wait for an opportunity to exchange it for a Metal Attribute or Earth Attribute Demon Core for the Golden Armor Rock Turtle and Golden Winged Tiger to consume when they advance to Third Order.
As for a Thunder Attribute Demon Core, those were too rare and couldn't be exchanged with a common water attribute Demon Core.
Furthermore, for Wood Charm, which was a Spirit Monster that cultivated extremely quickly, consuming a Demon Core would be a sheer waste.
