My human SIM turns into a Centaur whenever she bathes or showers

You probably have some badly-made Custom Content.  The "horse body" from RoseSims is known to be glitched in this way; though I have another piece of such content that I have not been able to track down, you should be able to find and remove this particular piece - either as something like "horse body all ages all genders" or as "NewMesh_rose_sims3_body014.package"; this would be in your program folder, C:\program files\electronic arts\The Sims 3\mods\packages (or C:\program files (x86)\electronic arts\The Sims 3\mods\packages" if you're on Windows 8 or suchlike, ie if the folder "Program Files (x86)" exists. What's happening is that this CC is tagged wrong, and as such is being erroneously selected as acceptable for "Naked" outfit for some Sims, in this case, yours.  I wish you luck solving this problem 🙂