Throat Wolf is a non-existent card. Shortly after the release of the first large expansion set, Legends, a couple of players on usenet decided to pull a prank and start rumors about this card. Since this was before the ubiquity of the Internet, many players didn't have access to full card lists and spoiler lists, and so had no way of knowing whether this card was real or not. Later, the term was used in a variety of in-jokes, and Magic developers occasionally use it as a temporary name for a card in development.
Various versions circulate on the internet, many with really obscure or rule-technically impossible rules text. The main ability that seems consistently attributed to the throat wolf is effectively 'double first strike' or 'firstest strike', worded in various ways.
In an article in the Duelist #9, Throat Wolf was described as 5 colorless mana for a 4/4 Summon Throat Wolf. The abilities read, "Firstest Strike. Throat Wolf may attack during your opponent's untap phase. Attacking does not cause Throat Wolf to tap." The flavor text read "The most feared of Dominia's mythical creatures, the Throat Wolf is wylie and unpredictable. Its incredible speed and attack abilities have prompted many net.wizards to wonder if it is real. Ask the Throat Wolf, he'll tell you..."
In the short story Chef's Surprise by Sonia Orin Lyris in the Magic: The Gathering anthology Distant Planes, the main character, Asmoranomardicadaistinaculdacar, works as a chef for the demon Vincent, Lord of the Pit. All of the meals she cooks for him are actual cards from the game (for example, atog pate on honey-soaked ironroot bark; Atog and Ironroot Treefolk are both creature cards) except one: Barbequed Throat Wolf ribs.