Based on some mythological analysis I've seen, some people think it is a Roc, the famous mythical bird. The Roc is a huge part of sailor/pirate lore, and was known for... wait for it... picking up baby elephants (Zou reference).
Also, no one has explained why the Kozuki crest is a bird, yet.
For a long time, I've thought that Rocks possessed the Cerberus Fruit, which let him eat two other DFs, and passed it on to Blackbeard.
Now, I'm wondering if Rocks had the Mythical Roc Fruit.