We wrote to The Bugman at http://www.whatsthatbug.com to help us figure out what species our cool caterpillar was, so we can look for the adult butterfly or moth on our future outings. Here is his response:
Hi Katy and her dad,
While we are unable to provide you with a species name, we can provide you with the genus for this horned Caterpillar. It is in the genus Charaxes, a group of Brush Footed Butterflies found in Africa. According to the Butterfly Corner website, there are over 180 species of Charaxes. Members of the genus are also found in India and other parts of Asia where they are known as Pashas or Rajahs. While this is not your species, you can see the similarities to this Madagascar native, the Green Veined Charaxes on the Online Fieldguide website.
This is an adult Charaxes jasius