<Window.Resources>
    <local:Source x:Key="src"/>
</Window.Resources>
<StackPanel>
    <Menu>
        <MenuItem Header="Animals" ItemsSource="{Binding Source={StaticResource src}}" />
    </Menu>
</StackPanel>

public class Source : ObservableCollection<object>
{
    public Source()
    {
        //Spiders
        Add("Golden Silk Spider");
        Add("Black Widow Spider");

        //BigCats
        Add("Jaguar");
        Add("African Wildcat");
        Add("Cheetah");

        //Amphibians
        Add("California Newt");
        Add("Tomato Frog");
        Add("Green Tree Frog");
    }
}

public class Source : ObservableCollection<object>
{
    public Source()
    {
        
        Add("Golden Silk Spider");
        Add("Black Widow Spider");

        Add(new Separator());

        Add("Jaguar");
        Add("African Wildcat");
        Add("Cheetah");

        Add(new Separator());

        Add("California Newt");
        Add("Tomato Frog");
        Add("Green Tree Frog");
    }
}