From A Wiki of Ice and Fire
Shadowcats are fearsome predators, but will also scavenge. They have thick black fur with white stripings, in size they are between real world tigers or mountain lions.
It's said that shadowcats can smell blood six miles away. They'll eat every shred of meat from a kill and will even crack the bones to get at the marrow.  Shadowcats tend to not attack living men unless very starved. 
Varamyr Sixskins, a diminutive wildling and a skinchanger, controls three wolves, a snow bear, and a shadowcat. When he wanted a woman from the villages, he would send his shadowcat to get them. His shadowcat used to fight him violently when he would slip into its skin. 
A Game of Thrones
A Clash of Kings
While making his way through Skirling Pass, Jon Snow watches a shadowcat stalk a ram, flowing down the mountainside like liquid smoke until it was ready to pounce. Jon wishes he could move as sure and silent as that shadowcat, and kill as quickly.
A Dance with Dragons
|“||A low rumbling growl echoed off the rock. Shadowcat, Jon knew at once. As he rose he heard another, closer at hand. He pulled his sword and turned, listening. ||”|
References and Notes