Is it advisable to create multiple classes within a .cs file or should each .cs file have an individual class?

For example:

public class Items
{
    public class Animal
    {
    }

    public class Person
    {
    }

    public class Object
    {
    }
}

Dodging the fact for a minute that this is a poor example of good architecture, is having more than a single class in a .cs file a code smell?