Twilight

Twilight is a panthara kitten.

Source
This item is available from the shop or the In-Game Store for $10 or €10. It was released on August 2, 2017 along with the Luminous Starseeker. They can both be obtained through a bundle for $30 or €30.

Pet Journal
This energetic kitten changes colors depending on its mood.

An energetic kitten that changes colors based on its mood.
 * In-Game Store

Demons Don’t Daunt This Dusky Darling - ''This cuddlesome little critter can’t wait to curl up by the campfire after your latest campaign. She’ll captivate your comrades with her chromatic color-changing coat and climb her way into the core of your collection…and she’s especially cunning in combat. Adopt this cute little creature today!''
 * Blizzard Shop