Lately I've been wondering whether Stan is actually an anthropomorphic purple crocodile in a human-like disguise. I have several pieces of evidence to prove that he is:
-His name is Stan Stanly Stanman. Doesn't this sound like a fake name that someone masquerading as a human would pick? It is widely known that crocodiles have a very bad memory for names, so choosing a name that is easy to remember makes a lot of sense. (It would be rather suspicious if he forgot his own name, after all)
-Listen to his voice in Escape from Monkey Island. He frequently makes odd noises and draws WORRRRRRDS out, sounding generally peppy. Now notice that he's in the warm tropical Sun in this game, while he's in the darkness in the other games. Seeing as crocodiles are cold-blooded, being in the Sun would make his body temperature a lot higher, making him a lot more energetic.
-In Monkey Island II, he sells coffins and does burials. This gives him an excuse to be on the graveyard a lot, where he can find lots of fresh meat (Notice the fact that a corpse half hangs out of its coffin in Rapp Scallion's crypt and the fact that only a bone was left of Largo's grandfather). Since he's in disguise, hunter for life subjects is a dangerous thing to do, as people could start looking in to why people where disappearing and discover he's an anthropomorphic purple crocodile. On the other hand, no one misses those who are already dead.
-Similarly, in Curse he does his business on a graveyard, again allowing him to easily eat some dead people.
-Between Revenge and Curse he spends three months in a coffin without food. For a warm-blooded animal like you or me, this is clearly impossible, but cold-blooded animals need much less food as they don't need to keep their body temperature up. (Just go to your local zoo and ask how often they feed the crocodiles if you don't believe me. You'll likely find the crocodiles are only fed once or twice a week)
-In Secret, when he and Guybrush are looking at the ship he's sold him, Stan has his hand on Guybrush's shoulder, clearly about to push Guybrush into the water and devour him. But shortly before Carla, Otis, and Meathook arrive, he makes a quick nervous exit. He probably heard them coming and realised he didn't want any witnesses to his feeding.
-In Secret he is known as Smiling Stan. Crocodiles are known for their big, toothy mouths that seem to smile.
-In Secret, we learn the only ones who ever go the better of him in a deal where the crew of chimps. It is widely known that chimps can easily defeat even an army of crocodiles (Just play Donkey Kong Country 2 if you don't believe me), so Stan was probably very scared of the chimps and just wanted them to leave quickly, so he agreed to a rather bad deal.
-In Tales, if you put a limb from DeSinge's lab on the altar, something eats it. Clearly, Stan's supreme crocodillian nose allows him to smell the limb all the way in town, and he gets so hungry he can't control himself, so he quickly eats the limb and frames that poor plant for it.
-In Tales, when the moths start eating his clothes, he runs inside, clearly afraid they will eat his disguise and he'll be unmasked in front of Guybrush as a crocodile. So he rushes inside and quickly puts on his spare disguise.
-Ever noticed that Stan seems to leave Flotsam island the moment DeSinge dies? Clearly, Stan always made sure the islanders blamed DeSinge for the limbs that he bit off, but with the good doctor dead, it would be very suspicious if limbs continued disappearing.
-Crocodiles always appear in pirate stories, yet we haven't seen a single one yet in Monkey Island. Clearly, this must mean a crocodile is hidden somewhere.
-Crocodiles never gesture when they talk to each other. Humans, on the other hand, do, so Stan overcompensates to make sure people don't start thinking he's a crocodile.
-At the end of Secret, when LeChuck punches him into the sea, Stan survives this, even though most people in the 17th century couldn't swim and the Caribbean Sea is full of predators. Crocodiles are natural swimmers and top predators, explaining his survival.
And there you have it. It seems very clear to me that in some future game, Stan will be unmasked and revealed to be an anthropomorphic purple crocodile.