Daisy always looked fairly different from Peach (I question the assertion that they were ever the same character), depending on which version of Peach you're comparing her to. Peach has undergone quite a few changes of her own over the years.
Princess Daisy appeared in at least one episode of the "Super Mario Bros. 3" cartoon show (MAYBE "Super Mario World," I don't quite remember). She was introduced as Princess Toadstool's younger sister, who ruled in "a nearby kingdom." In the episode, she was jealous of the other princess' kingdom, but she was far from evil.
BTW-In the cartoon, they were "Princess Toadstool" and "Princess Daisy," but after the revelation in "Super Mario 64" that the Princess Toadstool's full name is "Peach Toadstool," I assume Princess Daisy's name is "Daisy Toadstool."