I have seen several attempts to make monster rutting games, many however was abandoned and never taken up. Im talking abandoned for up to 6 years and still counting. there is one I thought that had some serious promise called monster cohabitation on pornplaybb.com but it never made it out of demo and the demo released 3+ yrs ago.
another was breeders of nyeplihym (cant remember the proper spelling. its hard on pc hardware and almost stands on AAA gaming so you need a decent gaming rig to play it. still waiting to see the naga/lamia in that one but its a good game even for its alpha stage and is updated regularly