This is based on the standard Crab Familiar from DND 5e. The standard stat block would be the same for the familiar itself, below are the quick construction guide for the bat.
Species attributes:
- Speed: 25 ft.
- Adaptation: Crustaformes come from various evolutions—decide whether you’re aquatic (only breathes water), terrestrial (breathes air) or interstitial (breathes both but needs to immerse themselves in water at least once a day).
- Natural Armor: You have rigid chitinous armor. When you aren’t wearing armor, your AC is 13 + your Constitution modifier. You can use your natural armor to determine your AC if the armor you wear would leave you with a lower AC. A shield’s benefits apply as normal while you use your natural armor.
- First Level Diverse Features: Bony Exterior, Grounded
- Recommended Diverse Features for 5th and 10th level: Natural Weapon, Blend In, Chemical Sensitivity, Climber, Digger, Directional Camouflage, Durable, Forager, Grappler, Natural Climber, Vice-Grip