B'Moth (The Master) is a god of the seas with a world-wide, but little known following. Probably an abbreviation of Beh'-Moth (known as Beh'-Moth the Devourer in the book Monstres & Their Kynde), thence Behemoth, the giant sea monster mention in The Bible. B'Moth wishes to send a horde of fierce animals to crush civilization and blot it out.
Probably another name for Cthulhu or one of his spawn like Leviathan.
Behind the Mythos
B'Moth was created by Bertram Russell during Lovecraft's lifetime, in "The Scourge of B'Moth" - it has been remarked that the story is practically a re-write of "The Call of Cthulhu"; Henry Hasse listed the creature among Mythos entries in a book quote in "The Guardian of the Book".