Concerning capra, my guess would be the general appearance of the animal - two little horns on the top of the head and a beard. I know the the others would also fit in that description (same for A. armata), but I imagine it was difficult to find a characteristic that was identifying it from the other known Acanthosaura at the time, so a name that was descriptive of the animal's appearance may have seemed appropriate. Hypothetically speaking, of course.

Jean F Déry