Snow bears

"Oh, that was bloody beautiful! Fucking Asher! You were pale as a snow bear's arse!"

- Croft

A snow bear is a species of bear.

"The Sword in the Darkness"
When Asher Forrester, Beskha and Malcolm Branfield reach Meereen, they meet with Croft of the Second Sons, who orders his men to apprehend them. Croft gives Asher the choice of losing an eye or a hand, as retribution for costing him his eye. Once Asher makes his choice, Croft plunges his knife into the tent support that Asher is pinned to and burst into laughter. Revealing that he was only playing a joke on them, Croft comments that Asher went as pale as a snow bear's arse.

In the books
In the A Song of Ice and Fire novels, snow bears are a direct analogue to real-life polar bears. They are said to grow thirteen feet tall and can be found beyond the Wall.