I have a serious idea for once what the update may have brought. I think you are all looking the wrong way. It is clearly not a block, mob, etc. I think it is a mapgen update. For instance, lava lakes are something the map generates at random. Could it be a natural house? A small circular cave that spawns a treasure chest in it? Anything that is part of the mapgen is a likely suspect. Keep an eye out for abnormal map characteristics if you want to figure out what is in the Beta update that is so rare.
I have a serious idea for once what the update may have brought. I think you are all looking the wrong way. It is clearly not a block, mob, etc. I think it is a mapgen update. For instance, lava lakes are something the map generates at random. Could it be a natural house? A small circular cave that spawns a treasure chest in it? Anything that is part of the mapgen is a likely suspect. Keep an eye out for abnormal map characteristics if you want to figure out what is in the Beta update that is so rare.