'I have some trouble thinking of the
Great Elephant as a Rook-equivalent piece.'
It has two full atoms, half of another, and a quarter of another; this puts
it already in the Rookish range, though maybe a bit weak. The partial atoms
are the forward parts, which must boost it to full Rookitude.
You have inspired me to write Captain Spalding Chess, of which the most
important feature is that one may find
an Elephant in one's Pajama.