The player can mount on a pig to turn him into a Tractor. When with a hoe in the off hand the pig will tilt the dirt and turn it into farmland. If the player has a seed on the off hand, the pig will plant that seed into the farmland, and if the player simply walks with the pig over a fully grown crop, the pig will harvest it and drop its items. Having a seed in the off hand will instantly replant that farmland.
To help with the movement with should use a Carrot in a Stick to move the pig to where you want.
If you feed a Wither Star to a pig it will evolve.
Wither Tractors are pigs that were infused with a Wither Star (by eating it). They don't need to be mounted or anything like that, they will function as a normal tractor is mounted, however they have a strange behavior when this happens, item lost and such may occur (its a way to prevent exploit of the system). They will walk around aimless or follow a player with a carrot in their hand.
They do tilt dirt, but don't plant in a empty farmland. You have to plant it first and then they will harvest it, drop the items, but no seeds will drop, they will consume all the seeds dropped to replant that farmland. They will always replant a crop of the same type that they harvested.
They are also faster then normal pigs, don't let it walk aimless, or your grass/dirt will all become farmland.