Some of the best stories I've read involve the main character(s) turn into something but they either don't notice and go about their lives including using new habits and movements (or lack there of) as they change, or notice that something is off (like not fitting into shoes or eating with their mouth and not with utensils) and brush it off as not important or rationalize it as being normal (why would they do it otherwise, etc) and even begin to identify themselves as their target transformation (thinking they are a cow or vehicle and don't need clothes, etc.).

One story that exhibits this rather nicely is an old story called Virus. http://roundhouse.wulfnet.net/stories/Virus.htm

As for rare species, insects are rather uncommon as are invertebrates in general. I think I've only seen two crustacean based stories, in fact. Octopi and squid are also rather hard to come by as are raccoons and skunks. Fish transformations (not just the obvious kind (I don't think I've ever read one about a seahorse)) are also a fave of mine especially when their minds change as well.

Thanks a bunch for the ideas! The seahorse proposal is pretty enticing... I may have to find some time to start writing!