The Pets category is only available as a boss objective.
There are several reasons pets and MM henchman are not available as standard foes; the biggest are probably that a) in many respects they're enormously weaker than "real" mobs (henchmen in particular, especially since they cannot slot for damage), b) they give no rewards, and c) in any case the generic spawn profile won't spawn anything except minions, lieutenants, and bosses unless it has no other options.
Aha... thanks, just the info I needed!
Okay, new thought-- is there a way to make a Fire/Fire Dom as a Boss that looks just like an Oni?
I'd like to add Oni (Ninja demon) to my custom enemy group, but I can't find an entry for them. Does anyone how where or how? I keep hearing they're in a Pets category, but I can't seem to find Pets.