Did you know Robin was only supposed to appear once and then disappear? Or that Catwoman used to really look like a cat (that's her above)?
3) Some of Batman's biggest villains were originally more animalistic.
Catwoman originally wore a furry cat mask that covered her whole face. And the Penguin was actually based on the cartoon penguin who advertised Kool menthol cigarettes in print, and also appeared on the radio shouting, "Smoke Kooools!" "Smoke Koools!" The original version of the Penguin was way more cartoony and hyperactive.
Learn more about The Dark Knight over at io9.