"Ah warn you, he's a coward..."
Fang was Hagrid’s pet and companion, an enormous black boarhound (PS8, GF13) with a booming bark (CS15). The boarhound is another name for the Great Dane. Boarhounds have a reputation for being gentle, even with small children, even though they earned their name by being used to hunt fierce wild animals such as boars and stags. Fang came off as a rather scary beast but, true to his breed, was at heart a coward. He seemed to like Ron particularly, even licking his ears when they first met (PS8). Fang lived in Hagrid’s cabin with him. Fang’s basket sat in the corner of the cabin’s single room.
Fang accompanied Harry, Hermione, Neville and Draco into the forest for their first detention. He isn't really much help, however, since he ran away from things. Fang demonstrated a little more gumption when Lucius Malfoy visited Hagrid's cabin. Then Fang growled menacingly (CS15).
When Hagrid was locked up in Azkaban, Harry and Ron looked in on Fang at least once. They took him along on another foray into the forest following the spiders, and once again the boarhound didn't prove to be worth much as a protector or guide (CS14).
"Fang" usually means a sharp canine tooth or similar feature in dangerous biting animals like carnivorous mammals or snakes. It's an ironic name because it suggests ferocity but its bearer is so gentle. It's almost the opposite of Fluffy, who really is ferocious, but has a rather diminutive name suggesting docility.
The protagonist of Jack London's novel White Fang is a wolf-dog cross called White Fang, who starts out wild but becomes domesticated.
In the films, Fang is incorrectly played by a blue Neopolitan mastiff. Thanks to Kelley Gary for identifying the breed for us.
Boarhounds/Great Danes are known for their enormous, even imposing, size and their surprisingly sweet and gentle nature. Just like Hagrid. -BB
According to Wikipedia, boarhounds/Great Danes have a life expectancy of just eight years. Supposing that dogs in the wizarding world age at the same rate as dogs in the Muggle world, this would make Fang rather old for his breed by DH - especially since he appears to be fully grown in PS. -BB