User blog comment:Onitatakau/Crocodile/@comment-3242083-20110117132047

Well, his scar is his most prominent feature and kinda resembles a "crocodiles smile". He also seems to have a fable for crocodiles, maybe he picked the epithet by himself.

While you have a point with the poisoned hook and the sandpowers, both are not visible from the start (the poisoned hook being more of a last resort "trump card" and the devil fruit powers don't alter his look permanently). Comparing it to other epithets, they usually center around one highly visible feature (straw hat, hawk eye, red hair,...).