I thought the main division between horses/ponies was the various breeds? Most pony breeds have pony proportions, pony heads, short legs in comparison to their body depth etc (so classed nothing to do with height but generally under 14'2 as a result?)
Even 15'2 connies generally have small pony ears and hooves and short legs in comparison to their bodies-hence pony strides etc
I don't know much about icelandics but for breeds like the american shetlands, aren't they built like small horses? Exactly the same just miniature version? Hence the name miniature horses?
Polo ponies are generally 15h and under, but the original polo breeds are criollos and arab/tb crosses? Pony breeds - hence the name polo ponies despite the fact that they make 15h.