"I want Fang."
-- Draco Malfoy in the Forest
The boarhound is another name for a Great Dane, a large German breed of domestic dog known for its giant size, which is bred for hunting deer, boar, and bears.
Hagrid's pet Fang is a black boarhound.
Like Hagrid, Fang looks much fiercer than he is. Fang accompanies Hagrid into the Forbidden Forest and also went with Harry, Hermione, Neville, and Draco when they served detention in the Forest. Hagrid told them that Fang is a coward, but this may have been more an oblique comment on Draco's behaviour than the strict truth about Fang. When Harry and Ron followed the spiders into the Forest and encountered Aragog, Fang accompanied them. When Hagrid resisted arrest late in Harry's fifth year, Fang was injured in attempting to protect Hagrid, but recovered.
Commentary
Etymology
In the 19th Century this breed of dog was known as the German Boarhound in English speaking countries. Some German breeders tried to change the name to 'German Dogge' or 'German Mastif' on the English market, in an effort to market the dog as a luxury breed rather than a working dog. However, as tensions between Germany and other countries increased, the dog came to be referred to as a Great Dane, named after the grand danois in an illustrated natural history guide in 1755.
Notes
According to 
Great Danes - Their Origin and Temperament, boarhounds (unlike other large breeds) do not slobber from their mouths, as Fang has been shown doing (PS8, CS7).