Dumbledore is not an Animagi, but it's possible if he were to become one his animagus would be a Phoenix. There is no definitive answer.
Voldemort was never given a reason to become an Animagus. We know his greatest desire is to be immortal, and being an Animagus would not help this. On the other hand, being an Animagus would not give him any special power in clearing his way to immortality. Killing curse is usually enough for him.
There is no evidence that he's actually an animagus. In fact, considering Snape's personality I would expect him to find a way to fly by himself without having to turn into a simple animal, like a normal wizard would.
Yes he does. Goku's full name is Son Goku, “Son” being the family name and “Goku” being the given name. It comes from how he was originally modelled after the Monkey King Sun Wukong from Journey to the West, “Son Goku” being the Japanese reading of “Sun Wukong”.
In-universe, when Goku arrived on Earth, he was adopted by Son Gohan (or as he's more known as Grandpa Gohan). Of course, the 'Son' is Goku's surname, which both his children, Son Gohan and Son Goten, also inherited. If by his real name you mean his birth name,then it is Kakarot,Son Of Bardock.
Gokus tail was cut off when he was still very young. It was cut it off By Kami to prevent him from transforming into an uncontrollable rampaging giant ape whenever the moon is out and full. They also destroyed the moon in case it wasn't enough. Gokus tail was cut off when he was still very young.