The Tentacle monster is a creature residing in the Gravity Falls Forest.
Its first official appearance was a flashback from Blind Ivan, in the episode "Society of the Blind Eye." When Blind Ivan was talking about the supernatural weirdness of Gravity Falls, Oregon, the Tentacle Monster grapped a truck loaded with lumber, and dragged it into the woods.
The full body and appearance of the creature have not yet been seen, only its huge tentacles that resemble those of an octopus, which suggests that it is very large in size. It seems to be very strong, as it was able to crush and lift up a truck, carrying large wooden logs.