Here's one character I can think of.
In this case he was in a manga adapted from the Anime.
N'Xau who was in Mobile Suit Gundam: The Origin, a manga adapted from the first show.
There were quite a few Fruits Basket characters not in the anime. Rin, Ren, Kureno, Machi, and Kakeru are the main ones that I can think of at the moment.