If would be an ability on the Diplomacy/Cooperation tree, with maybe the Empires being able to get monsters to join them more easily (or for less cost).    Maybe in battle some monsters would surrender instead of being killed, although the trade-off would be that you would not get the experience as if you killed them.  However you could sometimes impress enemy monsters outside battle.

The upkeep of the monster (in gold, food etc. - gotta keep those trolls from getting hungry! ) would have to be suitable based on the strength of the unit.  And maybe, if you can't afford to pay the monster what they want, they could go on a rampage through your Kingdom/Empire!   

So, what do you guys think?

Best regards,
Steven.