Lion's Tooth was Joffrey Baratheon's sword.
Lion's Tooth was a longsword proportioned to fit a twelve-year-old boy, made of gleaming blue steel, castle-forged and double-edged, with a leather grip and a gold lion's-head pommel.
A Game of Thrones
While out riding with Sansa Stark, Prince Joffrey Baratheon comes across Arya Stark and the common boy, Mycah, playing at swords with wooden sticks. He threatens Mycah, which leads Arya to attack him with her stick. Joffrey then threatens Arya with the sword, slashing at her, until Nymeria bites him to protect her. Arya throws Lion's Tooth into the Trident.
A Clash of Kings
A Storm of Swords
Within the hollow hill, Arya Stark misremembers the sword's name as "Lion's Paw". George R.R. Martin has stated that he was was trying to establish that the memories of his viewpoint characters are not infallible by having Lion's Tooth's name being misremembered (though this is accidentally attributed to Sansa Stark, instead of Arya).
Have no fear, lady. I am almost a man grown, and I don't fight with wood like your brothers. All I need is this. I call it Lion's Tooth.—Joffrey Baratheon