The zombie character will appear many times throughout a level, and other levels. For this reason, we'll build one zombie in the Viewport and, from that, create a Prefab. This may be reused as many times as needed. To start, drag and drop the zombie from the Project panel into the Scene, and set its Scale to 1.3 for all axes:
Adding a zombie character to the Scene and setting object Scale
Next, add a Nav Mesh Agent to the zombie, configuring it for a navigation mesh. Set the Radius to 0.26, fitting the Cylinder collider tightly to the character.
This volume approximates the character on the navigation mesh. If it's too large, the character won't fit through tight but passable spaces; if it's too small, the character will pass through impossibly narrow spaces.
Additionally, set the Speed to 0.6, and the Stopping Distance...