Rat

This is based on the standard rat 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: 20 ft. climb 20 ft.
• Skitter: You can move a distance up to your speed as a bonus action on your turn. You can use this ability once per short or long rest.
• Darkvision: Accustomed to darkness, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
• Climber: You have advantage on all Strength (Athletics) checks related to climbing.
• Storage Mouth: You can store objects inside your mouth. The total volume of the stored objects cannot exceed more than twice the size of your head.
• First Level Diverse Features: Digger, Grasping Tail
• Recommended Diverse Features for 5th and 10th level: Natural Weapon, Blend In, Canopy Cover, Climber, Durable, Digger, Enrage, Giant Leap, Hold Breath, Lands on Feet, Sensitive Strike, Stalker, Swimmer