1987: The year that Xenix (what SCO Unix was at the time) was ported to the x86, and the year that Minix was written for the x86. In other words, it's about a tie. But I'd say Minix is the original x86 UNIX since it was actually written for the x86, not ported to it.
"But I'd say Minix is the original x86 UNIX since it was actually written for the x86, not ported to it."
Good point--but then again I thought Minix didn't contain any original UNIX code despite being based on SysV.