I had 50-60 cows in a fence and they all dissapeared... is there anything in this pack that would randomly just make 50-60 cows go missing? and this is the 4th time I've started a world since they all keep crashing after 1 or 2 updates.

There is a general Minecraft feature called "cramming" that kills entities that cause too many collision updates. So that might be the reason your cows died.

Some Ars Nouveau Mod opponents would also kill the animals. Just a fence isn't enough to protect them, as some Ars Nouveau enemies can fly.

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