They can by eating, like fish that never see the sunlight. Grass also contains Vitamin D.

Another theory is that many birds and Mammals get their vitamin D from excreated oils which then are ingested while grooming themselves.

I also found a study where cows where covered and produced way less vit D, so it's definitely skin as well. Just because an animal is covered in fur, doesn't mean 0 sunlight reaches the skin.