This is based on the standard lizard 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.
- Skitter: As a bonus action, you can take the Dash action.
- Slippery: You have advantage on checks made to squeeze through tight spaces, typically Dexterity (Acrobatics), and checks made to escape from being restrained or grappled.
- First Level Diverse Features: Darkvision, Metabolize
- Recommended Diverse Features for 5th and 10th level: Natural Weapon, Bony Exterior, Climber, Durable, Hard Biter, Poisonous Bite, Poisonous Body