A hint on how to create some really weird mutations 
and have fun doing so :

El-Fish stores its fish as roe in binary files.
When you click on Restore, the software converts a 
roe file back into a fish.  But you can also convert 
almost any binary or ASCII File 
(a customer database, a budget spreadsheet, a 
health-benefits memo) into a fish.

Just copy the file with new extension .ROE. 
(Don't use EXE or COM Files.)

After loading El-Fish, Click on Restore From and find 
the fishy new file.
It will create one or more mutant aquatic life forms.  

I tried a file containing Earned Run Average statistics for 
American League pitchers, and as expected, the fish created 
was extremely ugly.