"Big Pony, Little Horse?" from Atlantic Horse & Pony
HORSE vs. PONY? It's a matter of semantics. The distinction used to be known in the English language only. (The word, some say, derives from the Celtic "ponaidh", meaning "small horse".)
Fjord horses, like Haflinger horses, Icelandic horses, Duelmener horses, etc., are called horses because the languages of the countries of their origin had only one word for equine. Any of these words translate in English simply as "horse".
Unfortunately, over the last 50 years or so, other West European languages have adopted the British word "pony" and introduced all the associated confusion with it. The generality, "Europe, Fjords are classed as horses..." is not true. It depends on who's talking and for what purpose the speaker is classifying.
In popular use, as well as generally in equestrian competition, "pony" is used as we do over here, purely as a distinction in height. Fjords measuring less than 150 cm (59 inches or 14:3 hands) at the wither, as well as Arabs or any other horse of similar size, are labeled "ponies".
But in Britain, the use of the word "pony" is twofold. Next to a statement of height, it is used as a statement of content, irrespective of measurement at the wither. "Ponies" in this sense have to possess characteristics of mind and body which are not found, or are strongly reduced, in other domestic horse breeds.
Almost all native horses of Asia are "true" ponies in this sense, as are many breeds in Eastern Europe, some in Scandanavia, many breeds native to the British Isles, and one in Ireland.
Ponies are superior to other horses in their capacity to survive and procreate without human interference. Moderate size is one of those things which help the animal to survive when feed is scarce and of low quality, but there is much more to it.
Exceptionally sound legs and feet (compared to other horse breeds), a sturdy frame (which enables true ponies to carry more weight for their size than other horses), powerful jaws to grind up coarse feed, a gut capable of digesting such fodder efficiently, and (for pony breeds from Britain and other places in norther climates) the ability to grow a good layer of fat for the winter and a dense winter coat, all combine to help the pony survive and raise foals without or with a minimum of human assistance.
Short legs make for a low point of gravity and surefootedness. Ponies are noted for fertility, longevity, quick wits, and the ability to take good care of themselves (which also benefits the rider, most of the time).
Pony-sized horses of breeds like Quarter Horses, Morgans, Arabians, etc., are not "native ponies". They do not share the native pony characteristics of body and mind to the same extent and, therefore, pony fanciers do not think that they should be called ponies at all, no matter what size they are.
Fjord horses, on the other hand, possess all the characteristics of a true pony. Yes, a Fjord is a horse, and a very able one like most true ponies. Every pony is a horse. Not every horse, even a pony-sized one, is a pony.