Platform, device version and operating system
What you were expecting to happen, and what actually happened
Craftable Zuul’Goth is currently classified as Boss. This is very likely a Copy&Paste error introduced by taking the Raid Boss Zuul’Goth as source, the Boss tribe seems to be limited to special encounters (e.g. Dungeon, which has Boss versions of troops obtainable by players). The intended tribe is probably Daemon, which would at least allow tribe synergies when using this troop.
How often does this happen? When did it begin happening?
Ever since the troop was released.
Steps to make it happen again
Keep skipping quality assurance on activities that have a history of being error prone.