My experience from adventure mode causes me to consider creature facing to be relative to where te creature last attacked or how it moved, various things apply, but large quadruped creatures attacking a companion opposit of me (classic flanking) makes it easier on average for me to hit hind legs, lower body and tails, while when it's trying to kill me I get better chances at upper body, head and front legs. When fighting very large creatures, you'll mostly get leg shots untill they fall to the ground. It is entirely possible for you to stand to the right and be able to hit right arm, right leg, head, upper and lower body - all valid targets, but having trouble hitting the left side.
I'm pretty sure of this since I almost never get a chance to cut off tails unless the creature is running or fighting a companion. It's just a bit confusing that you can't see which way someone is facing. This actually also influence your shield coverage too I think and having an archer shooting at you from behind is dangerous. In fort mode this can be exploited by making fortifications in reverse. A blank wall on the outside and fortifications on the inside. As your enemy charges in at the lure, like a puppy on a chain, your marksdwarves are going to have a fieldday.
As an aside, I found that as a human fighting bogeymen, I had most success cutting off heads, while as dwarf adventurer I did better cutting them in half at the lower body. It might be coincedence, but humans are 2x as tall and would easier hit heads, while dwarfs are more the same height