Fresh Animations Resource Pack (1.21.2, 1.21.1) gives an animation overhaul to the creatures in the default look of Minecraft. The aim is to make the mobs more dynamic and believable. This pack uses a feature of Optifine called Custom Entity Models (CEM) to add new animations. (It will not work properly without these features!)
Source: Fresh Animations Resource Pack (1.21.2, 1.21.1)
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