Windows RT. It was normal W8, except the W32 api could be accessed only by MS signed software so developers could use only metro.
Going back a bit you could include Windows CE alongside Windows RT if you liked. Windows Mobile as well. Is Windows Phone a version of RT?
Windows CE is entirely different beaste, but Windows NT 3.5/4/2000 beta had support for MIPS, DEC, ALPHA, PPC, ia64.
Windows Phone is some shared core components from Windows desktop (which RT is basically just an ARM build of) + its own stuff on top. The shared core has been growing with each release.
Windows could technically support loads of platforms, they just don't sell it.