You can reduce an FBX 3D model without manual retopology by using automated decimation tools in 3D software or online services—these tools lower polygon count by merging/removing vertices while preserving the model’s basic shape, no manual topology work needed.
Popular options include Blender’s Decimate Modifier (free, beginner-friendly), MeshLab, or online tools like MeshOptimizer. Most let you adjust reduction levels (e.g., 30–70% fewer polygons) to balance size and detail.
For a simple workflow, use Blender: Import your FBX, add the Decimate Modifier, set a "Ratio" (start with 0.5 for half the polygons), and preview to keep key features (edges, unique details) intact.