A zombie pigman spawned in the overworld when there is no nether portal built yet. I left my house and there was nothing there then I came back after about 5 minutes from mining and there was a pigman in the water nearby. In the mine, there was a "horde" of zombies and zombie villagers, even though there was no spawner nearby. I don't know if this has anything to do with it, but I thought it'd be good to mention it.