Krakens are very, very large animals that play a role in Artemis Fowl: The Time Paradox.
The Kraken is the world's largest animal that can stretch up to 1/3 miles in length. Contrary to belief, the Kraken is not a bloodthirsty animal. The People know that it is a docile animal whose main defense is it's size, shell and fat. Humans have made them seem like a giant squid, but it is more like a barnacle, "albeit a barnacle that could easily house an olympic stadium or two." Krakens have a lifespan of several thousand years, but because of environmental toxins, they are sterile, with only a few left. Usually they cause no harm to their surroundings, but once every century in their life, they ignite the methane in their shell causing a massive explosion and breaking apart their shell, allowing them to grow. Krakens are usually conicular but the seventh kraken that Foaly could not find was tubular.
At the beginning of The Time Paradox, Foaly only has knowledge of six living Krakens. He monitors their status with tiny light-up transmitters. When one of these transmitters begins to malfunction on the oldest of the creatures, Holly Short, now on Kraken Watch, is sent on a simple above-ground mission to check on the Kraken and pin-point the cause of the malfunction. While flying in the air above the Kraken, Holly notes that humans have mistaken the stationary creature for an island. They have built many buildings and businesses on the back of the Kraken, one of which being a sauna. Before Holly goes underwater to check on the transmitter, she does a thermal scan over the "island" to make sure no one is on it. She does see one red blob, but decides that someone simply left the sauna turned on. She flies down, shows Foaly the problem, and the centaur immediately tells Holly to get out of there. He explains that generally Krakens only live long enough to explode off their skin once, but that this one must have lived long enough to do it again. He says its skin will explode, shrapnel spreading everywhere. Just as Holly is going to leave, she notices four men emerging from the sauna; their heat signatures had been merged with that of the sauna's. She disobeys orders, diving back down to rescue the men, powering them off their feet and into the ocean with a few Neutrino 3000 blasts. She herself barely misses being injured by the explosion, later reasoning humans won't think anything of the explosion, as they are proficient with creating stupid stories to rationalize random explosions.
It is then that Holly gets a message from Artemis Fowl saying that his mother is dying and needs help. When Holly arrives at Fowl Manor, he lets slip that he believes there are actually seven Krakens in the world. Puzzled, Holly begins to question him, but Artemis quickly changes the subject.
In one of the last scenes in the book, Artemis is near a cliff, The Little Sister, with Opal Koboi chasing after him for the Silky Sifaka Lemur, Jayjay. Just as she is about to harm him, he takes out his laser pointer and aims at the rock around him. It starts to shake and then explode, burying Opal and narrowly missing him. This turns out to have been the elusive seventh Kraken; irritated by the laser pointer, the methane within its shell exploded, resulting in the Kraken shedding skin, before sinking into the sea, swimming away.