I'd go with the following:
Certain classes of animals will perform a coup de grace. This may be because of inbreeding, training, or other reasons. However, each animal within that class will _always_ coup de grace before going on to another opponent; it's the way they work. Chew until unmoving, then bite throat out. I'm thinking attack dogs.
Certain other classes of animals will beat on someone until they appear dead and then move on. Note the "appear": either being unconscious or having a good bluff check would work. I'm thinking bears.
Adding the "fiendish" modifier complicates things, because a fiendish animal would have two sets of motivations: the base animal and the fiendish part. The question is, does "fiendish" have its own motivations when it comes to coup-de-gracing someone. I, personally, don't think so; but that's me. I would have a fiendish animal act the same way as a normal one. YMMV.
Certain classes of animals will perform a coup de grace. This may be because of inbreeding, training, or other reasons. However, each animal within that class will _always_ coup de grace before going on to another opponent; it's the way they work. Chew until unmoving, then bite throat out. I'm thinking attack dogs.
Certain other classes of animals will beat on someone until they appear dead and then move on. Note the "appear": either being unconscious or having a good bluff check would work. I'm thinking bears.
Adding the "fiendish" modifier complicates things, because a fiendish animal would have two sets of motivations: the base animal and the fiendish part. The question is, does "fiendish" have its own motivations when it comes to coup-de-gracing someone. I, personally, don't think so; but that's me. I would have a fiendish animal act the same way as a normal one. YMMV.