Seems to me the brown eagle is rather rare, though I think it was in more than one set. Also the dark green regular birds. The green caiman has only been in one (rather expensive) set, though it's a still available set. And any of the brightly colored animals from the free game series that are not recent -- kittens, frogs, etc. The recent ones like koalas are not hard to find, but those have definitely only been in one set. And there's certain dogs that are pretty rare.