Animal ghosts

If ghosts really exist, where are all the animal ghosts?

Okay, occasionally you hear about a ghost horse, but it is usually being ridden by a ghost human. Or once in awhile a ghost dog, I guess. Are there ghost cows? Ghost chickens? Ghost lobsters? Ghost ants? If there are ghost ants, the world must be pretty crowded with kajillions of 'em. Wouldn't that drive mediums crazy?

I hope there aren't ghost spiders. That would just drive me over the edge.