My pony has one too he is certainly not fast, the old wives tale i was told by my vet is that when they are foals and in the womb the hind leg rests on the neck therefore causing the indentation!! Not sure how true that it is but kinda makes sense.
HI
I'm new here but thought i'd ;let you know i have bought one of those of ebay and it works wonders, i use a mark todd roller only about £24 and the clips all fit through the rings. My horse goes really well in it.
Lisa