Panthers are large feline creatures native to the Summer Isles.
After actions taken by Malthar Xaq and other princes of the Summer Isles began a new era of exploration and trade for their people, a fashion developed amongst the dragonlords of Valyria for panther cubs from the islands.
Anatomy and physiology
A Game of Thrones
When first laying eyes on Khal Drogo, Daenerys Targaryen thought him as graceful as the panther in the menagerie of Illyrio Mopatis. Catelyn Stark also thinks of the sellsword, Bronn, to be like a panther in terms of how he moves during a fight.
A Clash of Kings
A Dance with Dragons