I'm a native English speaker and I've never heard of a word that applies to something that is neither a mountain no a hill, but in between. Where I live (not in the US anymore) there is an area of the country that has what appear to be mountains - jagged, pointy peaks, steep sides - but are quite low in elevation, between 200 - 500 meters to the peaks. They are in no way hills because hills are generally rounded or smooth or somewhat flat on top, but they are far too low at their peaks to be 'mountains'.

So, what would you call them?

I would say the terms "hill" and "mountain" are both rather loose and blend into each other. We think of hills as being softly rounded, but that doesn't have to be so. Perhaps you could either define them by their characteristics (a rocky hill, a small peak, etc) or they may fall under other categories, like a butte, a ridge, an escarpment, a crag, etc.

Yep, that's what it comes down to. It seems rather odd that English, with one of the largest vocabularies of all languages, has no word for something so common. The other words all have specific meanings, and that leaves the two, hills/mountains + an adjective. It's just not right, it's English, after all.