Free tools for reducing prefab 3D models (simplifying geometry or lowering polygon counts) include Blender, MeshLab, and Simplygon Free.
Blender has a built-in decimation modifier for manual or automatic simplification, supporting common prefab formats like FBX. MeshLab excels at batch processing and cleaning meshes (e.g., removing duplicate vertices). Simplygon Free focuses on automated LOD (Level of Detail) generation, useful for game engine prefabs.
For Unity/Unreal users, try Blender’s direct import/export or Simplygon’s plugin to keep your prefab workflow smooth without extra steps.