Most map scripts like to put forests in clusters, without random clearings in the middle of the forest. But most resources can't start the game with forests on top of them. So when you see a bunch of forest all around and a random single non-forested tile in the middle, it's probably a hidden resource. Horse, copper, iron, something like that. When that non-forested tile is in the BFC of the default capital, it tends to be horse or copper; this has to do with how the game selects starting locations for players and tries to put them near horse or copper.
So as I said, that tile one south of the pigs is a very suspiciously unforested tile - "why doesn't this tile have a forest on it?" Probably because it had horses or copper; if you say there was horses nearby, I'm guessing it was that tile.