I was able to follow the link. This "costume" would be useful for any discussion of biomechanics!
As for my own - the only time I ever dressed up was as Red Riding Hood (I was riding in a split skirt, bodice/peasant blouse, and a sparkly red hooded cape, carrying a basket) - and I dressed the horse as the wolf (using black quilt batting (to wrap the neck and tail), grey hair/body paint, and a scrap of material attached to the bit to make fangs.
The scariest costume I ever saw at a show was a scarecrow (rider) in a cornfield (horse). Several of the horses were freaked out by it! The next year she used fewer cornstalks - so people and horses could better see the horse in it, lol. See at the left of this pic -
Recently someone showed me this video - http://www.youtube.com/watch?v=HeTOGsKjm1w Awesome.