
Horizontally-scrolling shooters that look and function somewhat similarly to typical vertical shooters, with a distinction between air and ground enemies.
These seem to be pretty rare. The only ones I know of are:
Progear
Akai Katana
Ironclad (though sometimes it uses a flat horizontal perspective)
Prehistoric Isle 2
Battle Traverse
I guess Break Thru counts as well, though it's a car-shmup so there's no air/ground enemy thing.
Not counting isometric games like Viewpoint or Zaxxon, though.