Fang appears only in Year One: Philosophers Stone, when Harry Potter, Ron Weasley, Hermoine Granger and Draco Malfoy have detention by Hagrid in the Forbidden Forest. When Voldemort attacks Harry and Draco, he goes with Ron, Hermoine and Hagrid to save him. You can use Fang to dig treasures, Lego Coins and several other things.
After that, Fang goes with Harry and Ron in the Forbidden Forest when Hagrid gives the hint that the must 'Follow the Spiders' in Year Two: Chamber of Secrets. You need him several times to dig in the ground.
In the DS-version, Fang only appears during the intro-cutscenes of 'The Forbidden Forest' and 'Spider Hollow'.