Enchanted Onyx

Enchanted Onyx teaches you how to summon an Onyx Panther, a house cat companion. The Onyx Panther awards the Feat of Strength.

Source
This pet was only made available on Korean realms, and was mailed to characters as part of a promotion.

Pet Journal
The onyx panther is invisible at night, like a stalking shadow, and disappears with the dawn.