I'm late to this but you might be surprised at my suggestion: Filipinos.
They will remain loyal, it's legal to drug them to get them to behave, and with enough trust they will actively fight for you. I've had three Filipino love slaves in my time and with proper training I managed to turn them into the best pit fighters I've ever had, the love after they crush an opponent's skull was nothing short of fantastic. However I don't suggest putting them in animal bloodsport leagues, pitbulls are dangerous no matter how well you train your Filipino.