Eating Lakius triggered the regeneration capabilities of the Black Dog Barghest. Not only that but its name also changed and its HP bar display has turned from a normal HP bar for bosses to a Raid Boss HP bar. Normal Boss HP bars appear much bigger than normal HP bars but raid bosses have their HP bars appear twice as big and have a stylized border depending on what kind of monsters they are, whether they are monsters from fiery lands or if they are monsters that control nature.
The Black Dog Barghest, now turned to its new name "The Black Calamity" has a misty black HP bar border. It's kind of similar to what the monster is in right now.
Despite the name and its transformation to a Raid Boss, it isn't getting bigger. Perhaps, it was due to the fact that it had just transformed that it hadn't turned big. But that doesn't change the fact that it has turned into a raid boss. Now, we have to kill it before it gets more troublesome to eradicate.