The Zombie Survival gamemode will automatically make these when 5 or more zombie spawn points are grouped together.
If you wish to place them manually so humans have a hard time spawn-camping, the name of this entity is " zombiegasses '.
Create a new entity
Double-click it in the 3D view
A new window should appear.
Type " zombiegasses " in the drop-down menu.
Press Enter.
Lasagna tastes good?