Behemoth are large, fabricated sea creatures, similar to kraken. One of them is used by the British to destroy two German ships, the Goeben and the Breslau, which are guarding the Ottoman Empire and the Tesla cannon there. Deryn plants vitriolic barnacles to eat the kraken nets and allow the behemoth to pass unhindered.
the Behemoth was made to be a companion creature to the Ottoman ship Osman which was a gift from the British, and both were supposed to be given to the Ottomans. Before the war started Lord Churchill "borrows" the Osman to keep it out of Ottoman hands during the war. In a coffee shop in the Ottoman Empire, Alek, Bauer, and Eddie Malone see a shadow puppet play about the behemoth. The Ottomans are very upset about losing the Osman, and it nearly leads to them joining the war on the side of the Clankers.
Alek thinks that the behemoth looks like "something dragged up from the deepest ocean." It seems to have life-threads from an anglerfish.It pulls ships apart with its huge tentacles, and has a huge mouth with large teeth. Alek also notes that the tentacles are only a small part of the behemoth, and that losing a tentacle does not affect it in any way.
Currently, there is only one known behemoth in existence.