I don’t think hordes work like that. I believe that the Zs floating around on the overmap are spawn points. The zeds move around at an overmap level by despawning after a time, and respawning elsewhere, so each time one spawns, it’s actually a different monster than when it disappeared.
In order for that to work, the game would have to persistently keep the existence of that one monster, which seems like a lot of overhead, including keeping it from despawning with the rest of the horde, if that is the case.
I could be wrong, though. If the damage on a zed stays even after it moves around in a horde, then it can probably be tracked like that.