Populous has some neutral objects like:

- wildmen (randonly roam in the map, but can be converted)

When creating maps in Populous World Editor, you can add.:

- neutral angel of deaths (which will not atack)

- neutral prison (as strong as any other prison, but takes no side, so any tribe can atack it)

- neutral buildings

- neutral vehicles (which will take a side once a unit of a tribe gets inside)