Why is White Horse not a Horse?

I am not exactly sure what you mean by white horse. Maybe you should elaborate more with your discussion, and people will be able to respond, because they know what you are talking about. Are you referring to a horse that everyone should be aware of? Or are you referring to a horse that is called white, but is actually another color? Make you question clearer, because I have no idea what you are talking about, and probably others do not know either.

There's a piece of Stone Age artwork carved into the side of a hill in the South of England (Wiltshire, I think, going from memory. It consists of a series of trenches filled with chalk in the shape of a horse, and it's called the Elongated White Horse. Would that be what you've got in mind? There's also a more modern one (carved in the early 19th Century, I think) near Weymouth in Dorset.

Hmmmm, I have a white horse. He looks like a horse to me. 4 legs, a mane and a tail, velvety muzzle, eats carrots and hay, whinnys, oh and ya, he produces plenty of manure. Yup, he's a horse alright. And he's definately white. So.... I give up. What's the answer?