The correct form is to sum up the legs involved. So two of the eight-legged creatures whose plural is in doubt would be hexadecapods. And if you have an octopus

The correct form is to sum up the legs involved. So two of the eight-legged creatures whose plural is in doubt would be hexadecapods. And if you have an octopus eating a kangaroo you'd then have a decapod, and so on.