During Pain's destruction of Konoha, he fought Naruto. When fighting Naruto, Pain hurt Hinata which made Naruto get mad. After a couple of seconds of being mad Naruto started to transform into Kyuubi.
How many tails did he transform to? I've heard and seen people say that he only got six tails, but I always thought he got all nine of them. Or almost got nine, until his dad stopped the transformation.