Marten

Supposedly the only monster that can kill a basilisk by eating a herb that neutralizes a basilisks venom. A man running a side show states that a basilisk is the most venomous and dangerous monster alive, even more dangerous than a Dragon.

“The one and only animal that doesn’t fear the basilisk is the marten. The marten, when it sees the monster in the wilderness, runs as fast as it can into the forest, looks for certain herbs known only to it and eats them. Then the basilisk’s venom is harmless, and the marten can bite it to death…”

Excerpt From: Andrzej Sapkowski. “The Time of Contempt.” iBooks. pg 113.

This occurs when Fabio Sachs is showing Ciri the city of Gors Velen. Ciri goes on to correct the man showing the "Basilisk" that it is actually a Wyvern.