A GHOST boss, or just called a "ghost boss" is a boss that has spawned off the map. These bosses cannot be reached by conventional means.
To know if a boss is a ghost boss, DFProfiler's Bossmap will tell you.
The ghost hound is the most common form of a "ghost" boss. As far as we know, the ghost hound is likely the only ghost boss spawn in Dead Frontier.
Almost everyday, a devil hound will spawn somewhere in the inner city. There is a chance that the devil hound will spawn in an unreachable area and will count as a "ghost" hound.
Yes there is! If you are in the Dead Frontier Series Discord, you can go to the #df1-bot-commands channel and be added to the 'dh' role to be notified about the devil hound spawns and if they are ghost hounds or not.