An interesting word.  We usually use "fantastic" to mean great (like a great window cleaner).  We continue to hear that sense of the word when we read it in the poem, but the dominant sense of "fantastic" in this line is strange or unworldly (as in "literature of the fantastic"), a now less frequently used sense of the word.  So the greatness of the crab is undercut by a sense of its oddness.  Another instance of the chain of associations around strength and vulnerability (compare oddly muscular).