Oink is the Doctor's pig. He first appeared in the Dragon Planet when the Doctor rode him through the jungle.
He was seen hanging around in the TARDIS in Prison of Monsters and the final part of Mission to the Sun.
The Doctor made him sonic sometime before series 2. In Journey to the North Pole Oink is seen flying above the ocean with his new sonic abilities.
The new sonic version of Oink can be seen in the background throughout Planet of the Weeping Angels.
The Doctor used Oink to fly after a plane in part 2 of The Answer to Death. It is unknown what happened to Oink after this.