Answer:
What word is a plural, but when you add an "S" to the end, it becomes singular?
White fat and fluffy, May even look like a puppy. Seems heavy but is light, Floats about without a worry, But if it turns dark you have to hurry!
When I live I cry, If you don't kill me I'll die.
I can be quick and then I'm deadly, I am a rock, shell and bone medley. If I was made into a man, I'd make people dream, I gather in my millions By ocean, sea and stream.
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
I am owned by Old McDonald.