This is based on the standard weasel 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: 30 ft.
- Ferocity: As a bonus action, you can enter a ferocious form that lasts for one minute. While in your ferocious form, you have advantage on attacks made with your natural weapons. If you reduce a creature to 0 hit points or less with an attack, you can use your bonus action to make another natural weapon attack.
- Keen Scent: You have proficiency in the Perception skill.
- First Level Diverse Features: Digger, Enrage
- Recommended Diverse Features for 5th and 10th level: Natural Weapon, Blend In, Canopy Cover, Climber, Darkvision, Durable, Giant Leap, Grasping Tail, Hold Breath, Lands on Feet, Sensitive Strike, Skitter, Stalker, Swimmer