This may or may not be known, but I've stumbled on a way to at least make your own forgotten beasts.
Since there was no difference in material properties for organs other than tile/color, I combined them all into a generic material called offal and pointed all the organ tissues to this new material. The point was to change the meat name of this material to yield 'sausage' when butchered. It worked great for my purposes.
However, when I generated a new world, I noticed error messages that it could not generate random creatures because these materials were missing (obviously hardcoded to use those materials). It did, though, grab the first creatures (in the order they appeared in the file) from the first (alphabetical) creature file and made them forgotten beasts (my first file happened to contain the fishes, which is why it was easy to notice). It did not generate night creatures, demons or overland titans.
Apparently, as long as you rename your materials (if you want to still use the default ones) and point your tissues at them, then put whatever creatures you want as forgotten beasts in the first creature file with enough of them to cover the count, you will at least be able to have your own forgotten beasts in your worlds.
Of course, there's more testing to be done, but I figured I'd turn everyone loose on it and see what you come up with.
(Edited for clarity.)