Pig is not a pig. Pig is an owl. A very small, exciteable owl that Sirius gives to ron as compensation for having rid him of his rat, Scabbers. Ginny names Pig in Book 4, and the name sticks.
And... well, Pig is an owl. What more is there to say about him? (her?) Hedwig doesn't like him/her and he/she tries very hard to please. He/she even likes running errands.
ron still doesn't treat Pig very well though, probably because he/she is too enthusiastic and ron is not. And because Pig is cute and ron is not. *sigh*