I supposed we can A/UX to the list. So, its A/UX -> AIX -> OS X Server. I do admit an iOS Server would be an interesting product. I guess it would be all containers all the time.
(The chronology of Apple *nix is correct, but they are very different products. A/UX for 68k. AIX for PowerPC is an IBM OS. And OS X from NextStep.)
Before the Xserve there was the Apple Network Server https://en.m.wikipedia.org/wiki/Apple_Network_Server