Transylvania doesn’t mean “forest on the other side”. It means “on the other side of the forest”. In this case on the other side of the forest from Hungary. So Cisylvania is Hungary.
You might consider cisalpine gaul and transalpine gaul. Regions where the Gauls lived either on “this side” (the Italian side) of the Alps, or the France side.
This is the same etymology as Transylvania, or, the Forest on the Other Side.
This begs the question, where is Cisylvania?
Transylvania doesn’t mean “forest on the other side”. It means “on the other side of the forest”. In this case on the other side of the forest from Hungary. So Cisylvania is Hungary.
Damn, I knew I should have looked it up to be sure first. At least we’ve discovered Cisylvania.
You might consider cisalpine gaul and transalpine gaul. Regions where the Gauls lived either on “this side” (the Italian side) of the Alps, or the France side.