Forgotten Beasts are a class of monsters from Dwarf Fortress. They are procedurally generated, thus they can come in many different shapes and forms. One might be a humanoid made from metal, another might be a giant spider that emits poisonous gas an yet another might be a giant bird with deadly secretions. There are also Titans, which are similar except where Forgotten Beasts are subterranean, Titans dwell on the land and in the sea. I am thinking about how to integrate such creatures into a D&D setting.
While there's no need for a specific origin for random esoteric creatures (yes, that's a reference to THE RANDOM ESOTERIC CREATURE GENERATOR FOR CLASSIC FANTASY GAMES AND THEIR MODERN SIMULACRA, by James Raggi, a most useful resource...), for "Forgotten Beasts" I'm thinking that they would be creatures created/warped by the weird radiations and extraplanar energies found in the underworld (or Underdark, Darklands, Khyber, etc). Perhaps these creatures are servants or creations of an evil or merely primal, elemental deity who has been lost to the annals of history, hence their name of Forgotten Beasts.
Continue to observe this post and I may post a few sample Forgotten Beasts.