If you evolve into a hostile alien race, with a need for slaves, I think it would be a cool feauture to enslave another species to serve you. You could have them build things for you, live in districts, you can kill them too.
Another cool idea I had was to force a species into bio-mechanical slavery, by manufacturing them into semi-robotic slaves for your own military. You might be able to this with animals, by using them as a cybernetic war machine in your divisions. You could sort of build a ‘union’ this way, combining different cultures together into one whole one.