first, there was Disney Bear. basically a plush created in the North American parks that features Mickey Mouse-shaped accents - on his face, his paws and butt. there's also a book about Mickey taking his teddy bear to the Magic Kingdom and wishing he'd come to life. they did create a meet & greet character to promote him:
but other than for rare, special occasions (which are likely to be done with completely, he can't be found in the parks.
I also have a couple of limited edition Steiffs of Mickey and Minnie holding their Disney Bears but I don't recall seeing much more in way of that.
anyway, Tokyo DisneySea basically took Disney Bear and transformed him (incredibly slightly) into Duffy and introduced him as a bear that Minnie had made for Mickey before he took a long voyage.
he's a huge deal over there. in addition to tons of merchandising and his meet and greet, they gave him his own show:
http://www.tokyodisneyresort.co.jp/tds/english/7port/waterfront/show_duffy.html
So now they're going to try to make him work in October at both Walt Disney World and
Disneyland, no doubt trying to play on his success with existing Japanese visitors as well as everyone else.
so be prepared for Duffy's Great Escape
perhaps coming to the Imagination Pavilion.