I tell you, that dragon's the most horrible animal I've ever met, but the way Hagrid goes on about it, you'd think it was a fluffy little bunny rabbit. When it bit me he told me off for frightening it. And when I left, he was singing it a lullaby."
-- Ron Weasley (PS14)
Norbert is a Norwegian Ridgeback dragon, hatched from an egg by Hagrid and raised for several weeks in the spring of 1992. Norbert outgrew his surroundings rather quickly and had to be sent to a dragon reservation in Romania, where they discovered that Norbert was actually female and renamed her Norberta (PS14, PS17, DH7).
Hagrid got Norbert’s egg from a stranger in a cloak in exchange for information about Fluffy the three-headed dog guarding the third-floor corridor at Hogwarts. When Harry heard the story, he realized that Hagrid had been tricked by someone who knew he wanted a dragon egg, which turned out to be Professor Quirrell who was trying to steal the Philosopher’s Stone for Lord Voldemort (PS16).
It's striking that Hagrid adopted Norbert and treated him with gentle motherly love in spite of his ferocious traits, and was quite upset when he had to be sent away. Quite a contrast to Petunia Dursley, who adopted Harry and lived with him for eleven years, but didn't even say goodbye when Hagrid came to fetch him. Of course in Petunia's eyes, Harry was in many ways just as strange and dangerous as a baby dragon.