Other mistake: When Rango is loading his revolver during the stand-off, he loads the bullets on the right of the revolver, however, the chamber is hanging from the left of the revolver.
Other mistake: When Rango and Rattlesnake begin walking toward each other and the clock strikes noon, there are only eleven chimes, not twelve.
Answer: TV Tropes mentions that the big eye might be an enormous alligator, because alligators are known to live in sewers.
Alligator eyes are different from what was shown in the movie.