If you'reasking about the green "Alien Cow" aka Bov1ne-09, the answer is Milktonium. It's gone now (limted edition) but maybe they'll bring it back next year.
To harvest cow on farmville you need to click it. Your character will then go over to the cow and harvest it.
Yes, it's faster than just clicking on each cow.
you can harvest these alien cows for their green milk, and get coins for them
If you put your cows in a dairy farm, then put a bull (they wonder ARound friends farms,) in it, when you harvest, a calf may be born.
No. Animals don't die on Farmville.
You can get cows on Farmville when your Farmville friends gift them to you or by adopting them when they wander across a neighbors farm and that neighbor posts a notification about it allowing you to adopt the lost cow.
You harvest feathers!
You can harvest horsehair from them.
No.
You can harvest goat's milk from them
No.
Horse Hair