This creature has human-level intelligence, and after spending enough time around humans as a pet, it learns to understand the human language. It isn't hostile and has no wish to attack humans. It hangs around humans because the humans will provide it food, shelter, and companionship.
It's unable to vocalise the sounds needed for human language, so speaking is out of the question. However, it can still read and write the human language and show the humans written messages. Unfortunately, the humans find it very difficult to understand what it has written. What the humans do understand of its writing makes them think this creature is not very intelligent.
Why would the creature not successfully communicate with the humans?