Factual error: When the beast itself moves, it bounds, but the sound effects make it sound as though the beast is running.
Factual error: In the book by Douglas Preston and Lincoln Child, the monster has been in the museum's subbasement for around seven years, making the lair filled with either partially or fully decomposed skeletons completely plausible. However, in the movie they change the duration to only six weeks. The lair they find in the movie with tons of skeletons doesn't connect with the time it took for them to become that decomposed.
Factual error: When D'Agosta and Hollingsworth are in the Santos Morales, they find blood on the walls and floor which is not only dry but also red. Dry blood is brown.
Factual error: When Margo and Lieutenant D'agosta walk into a lab, Margo asks one of the techs what the "soup" is, to which the tech responds Rhinoceros Megarhinos. No such species exists.
Answer: More then likely, the dog was too afraid of the Kothoga to attack it and when the Kothoga saw this, it let the dog live. The other dog was probably unafraid and tried to attack the Kothoga which prompted it to kill the dog; not out of self-defense, but, because there were still people trapped in the museum and the dog was impeding his attack.