I played minecraft when suddenly some flying pigs spawned. They look like small pigs flying on a chicken (they are relatively small so it's hard to see):
They fly down in a straight line and then disappear with an "explosion" effect, but no actual blocks are destroyed. When they fall into water, there is a "bubbles" effect instead. Also, they seem to be immune to attacks. This is one immediately before impact:
I noted that the pigs started to spawn at ~21:00 on 25. December, so maybe it is because of some holiday (presumably christmas)?