I would say the key point is that a "dandy" has his own aesthetic that endures....so there will be times when he looks "in style" and other times when he doesnt (after the fashion wheel has turned on).  But he has a very deliberate aesthetic that goes beyond brands and labels, as he doesn't care if something costs $50 or $500 as long as it fits into his aesthetic.

The clothes horse is more common, someone that buys (usually high end) something just to have it, usually characterized by "quality" items made to have no distinct personality and plays it safe when it comes to sartorial expression.  Generally his closet elicits oohs and aaaahs because of the labels, which brainwash many into thinking it looks better than it does.  No personal aesthetic, usually follows all the established 'rules' and would consider a pink tattersall shirt to be the limit of his sartorial extremity

interesting point about "Well Dressed Man"....I'd agree with that, tho I would also say that in many instances he is an aspiring "Clothes Horse" just without the financial means to properly be one.  I don't think WDM has the clearly defined aesthetic that Dandy has, tho that's not to say he doesnt have a look in mind that he's trying to reveal, it's just as not strictly defined.

What of, a clothes Goat ?

It seems to me that a clothes horse is self-conscious, whereas a dandy is self-aware; a clothes horse seeks approval from others, while a dandy is not concerned with what others think and dresses to please himself.

I think the clothes horse would be much more swayed by fashion. He would be likely to wear RTW shirt-tie combinations. He wants to be seen as being above reproach, but needs help from the mainstream to do so.

The dandy, by contrast, disregards fashion in favor of his own vision. He may have an awareness of fashion, but it does not dictate how he dresses. He would be much more likely to have items custom made for him in the exact shades and designs he envisions. His sense of style and taste are so refined as to seem innate.

We need to distinguish yet another type, and that is the fop. The fop is not so much a dandy gone wrong, but a clothes horse on steroids. Clothes may be custom, but are marked by a self-concsious, attention-grabbing flamboyance that strikes the observer as being overdone. In terms of personality, the fop would tend to be a buffoon.

FilmNoirBuff, your last post hit the nail on the head.  It is perhaps one of the best definitions of modern dandyism I have yet to find.

Neither has an appealing image for me   The Dandy wears clothes to satisfy his  conceit, combined with a desire, or need, for attention, while the clothes horse lacks discernment.  I like the idea of appreciating and enjoying fine clothes as highly well-crafted works of art.  In taking the time to assemble a wardrobe, and putting thought into what you wear each day, even if only to take the dog for a walk or take a trip to the supermarket, you show respect for the makers of the clothes, respect for their products, as well as respect for yourself and for those around you.