Can Swallow a Steel Nail
Don't know if this helps, but farmers use an item called a cow magnet. I've seen 'em first hand, and they're rather large, being 2-3" long and around 3/4" in diameter.
The reason they do this is because there's a possibility that a bovine could accidentally swallow a piece of steel lodged in their feed or straw. The idea is that the magnet latches onto the steel bit before it goes through the digestive tract, causing intestinal damage to the animal.
I may be incorrect, but I believe that cows have two stomachs, and the magnet, since it's ingested, would most likely reside in the first.
As far as animals swallowing nails, most any animal could probably do it, but I don't think it would be intentional and it's probably very rare (except for cows, of course ;).