Hmm. Let's go for a natural look. A brown Earth pony with short blondish mane, male. Wears glasses, somewhat rectangl-y ones (you can see another cartoonish version of them at http://www.livejournal.com/allpics.bml?user=katsaris ). For a cutie mark, a closed book with a symbol of silver pegasus wings on its cover. The idea expressed being he likes to write/read/imagine about the capabilities he doesn't have.

The name "Aristotle" doesn't fit as well now with the other ponies' names, so go with Elfwhinny. (which is a pun on Tolkien's Aelfwine)

If I left anything out, feel free to embellish. :-)

Thanks again!