Hellhounds of course, with lots of Petplay, knotting, neck biting [to show dominance], and maybe a bad end where you are collared and made into a submissive Dogboi [domestic canine-themed trap, maybe like the MGE one where they are maid-themed] and well taken care of, but still property of your Owner and milked regularly to make more pups.
also i'd love an enemy with a kiss mark theme, something like lust-stick or whatever. i don't know what type of monster such an idea would fit in best with but its definitely my biggest fetish and a scene about a girl pinning you down, mounting you, and methodically coating you in sticky glossed kiss marks would be heaven for me, her commenting how everyone will see her marks of ownership and know you were helpless to keep her from claiming you.