I've never seen any drawings of anyone other than Mickey by Walt. I even just checked the Illusion of Life book (the Bible) and it had nothing. I think all of the secondary characters were probably partially designed by Walt, but I think that Ub Iwerks probably had big hands into the creation of Minnie and Pluto. Donald, Daisy and Goofy I would almost guarentee were drawn by someone else first. Walt probably just had an idea and had others come up with designs.
I'm not saying any of what I just said is absolutely true, but I can't remember any drawings of any character other than Mickey that people have seen Walt draw. When I didn't find anything in the Bible I wasn't sure what to do...that book has everything.