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Originally Posted by Electric Turahk

Skulls? Where is anyone seeing a skull in his tail??

A number of people have pointed out that the ridges in the exposed region of the tail tip resemble whale skeletons; Google image search Shin Godzilla tail and you'll likely find at least one zoomed in shot of the region in question.
That being said, not sure the absorbing other monsters theory is too likely. Everything shown thus far indicates a "Japan vs. Godzilla" kinda movie, including the numerous similarities to the original. (trailer even shows Shin Godzilla attacking the same region where the original first attacked). I suspect they're supposed to be either whale bones that were fused into him during whatever created him (they are really pushing the scarred, burned, mutated look with this design) or they're scars that incidentally happen to look like something else.
As for the Godzilla monster A and B thing, those could just as easily be new figures of older kaiju that don't actually appear in the movie, or just variants of Godzilla himself.