I see where you're coming from now, Eddie. I was taking the word "Celtic" in your thread title in the literal sense. The timeline for UK Celtic history commences 500-400 BC, the ancestors to our Exmoor and Dartmoor ponies were certainly on the scene by then. However, I believe you are talking about the period in time prior to this - an early "Bronze Age horse" rather than a "Celtic pony"?
Whilst to the best of our current knowledge all horses descended from one common ancestor, there were several branches of horse that evolved from this point, eg the Mongolian horse was developing at the same time period in history as the Caspian. Since archaeological examination of the most ancient remains of UK horses cannot reliably link it to any modern day breed, one could theorise that this was one of the "branches" that didn't make the grade and died out?
However, I wouldn't discount your theory either. Unfortunately, the evidence to support or disprove it has been lost or is yet to be discovered (perhaps in one of your Irish peat bogs?). It is a fascinating subject.
PS. Merlin is still smarting from you describing his features as "dwarf like". In his head he is all of 16hh - he's only 2, got some growing to do . . .