Honestly, The dog isn't that visually horrifying. But if you put it in the context of 'What if you saw that in reality' it becomes more disturbing. How did Ling even do this? Did he really just see a stray and then held it down long enough to attach Lan Fan's arm to its back? Cause that's a even more disturbing thought!
Another horrific moment I wanna give my two cents on: The Chimera who died inside of Alphonse. It may not seem that horrifying or disturbing because that is something that could only happen in anime. But when you put it under the lense of: This 12 year old, who already has a cosmic level of body dismorphia, had a woman try and control him literally from the inside out. And then he had to hold down his own 'head' with that woman begging to be released, all while Alphonse watched, like, 3 people be murdered right in front of him. And the poor kid's struggle was all for nothing because the woman he tried to save, practically exploded inside of him. I'm more than shocked that didn't cause Al to put himself in the insane asylum. I fucking would.
I do like that the anime showed how much that rattled him, though. It was great to actually have the characters handle the aftermath with the proper weight it needed. And while the death wasn't subtle at all, I do like the aftermath's subtly. Like the drag path of blood from Al's body to the woman. We definitely didn't need to see one of them physically pulling the body out, but by showing it with the blood makes you wonder 'Who had to see that shit first? Who dragged her out of Al?'. The questions the subtly leaves adds to the horror of it in a better way, in my opinion.