Let us assume that we are settlers on some alien land and we run into a terrestrial, territorial pack carnivore among the local fauna. This animal has many of the traits needed for domestication:
- It forms a strongly hierarchical pack structure, much akin to wolf packs
- It is intelligent, on par with dogs, and in particular, the German Shepherd (a breed well-noted for its intelligence and trainability)
- It is carnivorous, but not exclusively so -- it seems to adapt well to feeding off table scraps as well as its normal diet of meat
- It can be bred readily in captivity, and takes on an even temper when raised in captivity without too much difficulty (the model for this is again the German Shepherd)
- It does not pose major husbandry difficulties in terms of disease/illness or specialized care needs
How large can this animal be before domesticating it for hunting and guard duties, as well as general companionship, becomes impractical for the settlers to pull off? You can assume that they have skilled animal trainers and breeders among their ranks.