Hagrid's pets

In the Harry Potter series of novels by J.K. Rowling, Rubeus Hagrid owns many pets, including some which are believed by the wizarding community to be impossible to domesticate. He seems to have an intrinsic attraction to dangerous creatures, which was a contributing factor to his expulsion from Hogwarts.

Aragog

Aragog is an acromantula and was raised by Rubeus Hagrid in 1942: in fact, it was for raising such a dangerous beast that he was expelled from Hogwarts (see Harry Potter and the Chamber of Secrets for more details). Aragog is currently living in the Forbidden Forest, where, with Mosag, his wife, he has sired numerous offspring. He is grey and blind.

Ron and Harry encountered Aragog in May, 1993 and were attacked by the numerous spiders living with Aragog (presumably, all of them are his children).

Aragog comes from aranea (Latin word for spider) and Gog (a legendary giant).

Fang

Fang is Hagrid's dog. Much like Hagrid, Fang is very large and often appears intimidating, but is actually friendly and rather harmless. Unlike most animals in the wizarding world, Fang appears to be an entirely ordinary dog. He is the most featured of Hagrid's animals.

Fluffy

Fluffy is a vicious, giant, three-headed dog found in the forbidden third floor corridor on the right-hand side. The only way to get past Fluffy is to play him music, but Fluffy wakes up the moment the music stops playing.

Fluffy is used as the first line of defense against potentional thieves of the philosopher's stone. As the somewhat-whimsical name suggests, Fluffy belongs to Hagrid, who got him from a Greek person he met in a pub.

In the book Harry Potter and the Philosopher's Stone, Professor Quirrell gets past Fluffy by playing a harp, while Harry, Ron and Hermione use a flute that had been given to Harry by Hagrid.

In the film adaption, Quirrell enchants the harp to play autonomously and it is still playing when Harry, Ron and Hermione arrived. The harp stops playing unexpectedly while they were still talking so they have to jump through the trapdoor to escape.

Fluffy is based on Cerberus, the three-headed dog from Greek Mythology that guards the gates to the underworld, hence the reason that Hagrid got Fluffy from a Greek person. Like with Fluffy, Cerberus was lulled to sleep by music, though this is more a testment to Orpheus' music skills rather than susceptibility of Cerberus.

Note 

In the film Hagrid says he got Fluffy from an Irish person instead of Greek: This is a mistake. Steve Kloves says when he wrote the script it said "Greek," but was changed somewhere along the line. J. K. Rowling was asked in an interview what happened to Fluffy after he was no longer needed to protect the Stone. She said Fluffy was released into the Forbidden Forest.

Norbert

Norbert is Hagrid's baby Norwegian Ridgeback dragon.

Hagrid secretly hatched Norbert in his hut from an illegal egg (the Ministry of Magic classifies dragon eggs as Class A Non-Tradeable Goods). Norbert grew at an amazing pace and was very nearly discovered when Draco Malfoy eavesdrops on Hagrid's conversation with Harry and his friends. Harry eventually helps send Norbert to Charlie Weasley in Romania during a midnight escapade up the North Tower in which Harry and his friends are caught and given detentions.