Mopy at http://www.mopyfish.net/ is a state of the art "fish"?
Based on the patterns of real fish it is basically a cyberpet for your pc.
It is not entirely portable as-is but it seems, after testing a little, that if you don't name your fish and do not try to earn "Mopy points" (I still don't get that) it doesn't seem to write anything anywhere.
I may be wrong so if someone would like to test it more thoroughly please God do.
The installer can be extracted with winrar, copy the BRAIN.DAT, MOPYFISH.EXE, MOPYPLAY.EXE, MOPYFISH.DAT and the HOOK1632.DLL to a folder and run MOPYPLAY.EXE.
Naming your fish creates an ini in the windows folder containing your fish name.