The easiest way to explain this is the horse's neck looks disproportinately short and overdeveloped on the underside.
This is bad for two reasons:
1) it usually means the shoulder is quite upright (which means short, choppy action in the front - not fun to ride); and
2) lack of musculature over the topline (which means the horse is carrying itself in a manner that makes it prone to break down)
Bottom line, while every horse is different, a ewe-necked horse is (in every book I have read) a pretty risky bet from a performance perspective.
Could really find a good definition but here --
ewe-neck (plural ewe-necks)
noun
Definition:
neck defect of horse or dog: a thin concave neck in a horse or dog, considered to be a disadvantage
But I still would say your horse is in fact, not 'ewe neck'.
