I figured it would be likely that people wouldn't believe me, but no skin off my back.
This is single-player, by the way. Been playing for a year, and never had any interest in playing on a server. No tricks involved, and nobody else in my household but my girlfriend (who has never played and doesn't have any interest) to set me up.
I'm guessing it is a bug. Never seen anything remotely like this in all the time I've played, but it instantly reminded me of the screenshot with all the sheep.
The chickens (at least most of them) were still there when I came back by the area on my way back from building the new outpost. Lots of eggs around too.
Encountered my first mushroom biome near the new outpost too, but it only consisted of two very small islands of mycelium. No giant mushrooms, but several mooshrooms wandering around the surrounding water.
Back on topic, bug or not, I can imagine any sort of animal breeding that isn't somehow initiated by the player would always run the risk of huge populations of animals eventually taking over the world. Kinda like mushrooms did, when they started multiplying, but at least that was limited to dark caves and the nether.
Seriously? Nobody else has run into anything like this?