This tiny clockwork construct rodent is built using bronze plating and clockwork gears, iron rivets, steel springs, and has a single enchanted aquamarine gem. 
The Tinkerer's Familiar Rat looks similar in many ways to a common rat, however it has a shiny bronze exterior, non-organic symmetry, many weird and sharp angles. The tail is long, like a rat's but segmented and prehensile.

The Rat is activated by inserting a small iron key into a keyhole in the back of the bird. Turning the key 10 times clockwise will wind the Rat up, after which it is activated for 1 hour. There is a DC 5 chance that the winding mechanism will break. Roll a d20 and add your Proficiency Bonus (for Tinkerer's Tools). On a failure, the winding mechanism must be repaired. Repairing the winding mechanism costs 50 GP worth of materials (springs, gears, fasteners, etc.) and can be completed during a long rest. 

Although the Tinkerer's Familiar Rat  is quite fast, however it's clockwork mechanism is quite loud, thus it has disadvantage on DEX (Stealth) checks

Your Tinkerer's Familiar acts independently of you, but it always obeys your commands. In Combat, it rolls its own Initiative and acts on its own turn. A Tinkerer's Familiar  can't Attack, but it can take other Actions as normal.

When the familiar drops to 0 Hit Points, it is destroyed, but can be rebuilt at a cost of 500 GP in materials and 1 week of Downtime.

While your familiar is within 100 feet of you, you can communicate with it telepathically. Additionally, as an action, you can see through your familiar's eyes and hear what it hears until the start of your next turn, gaining the benefits of any Special Senses that the familiar has. During this time, you are deaf and blind with regard to your own Senses.

You can't have more than one Tinkerer's Familiar  at a time. 

Finally, when you Cast a Spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its Reaction to deliver the spell when you cast it. If the spell requires an Attack roll, you use your Attack modifier for the roll.

Notes: Gnome or Proficiency with Tinkerer's Tools