How does one determine the correct plurals of "made up" words that have no given plural form given by its original canon source or creator? Is it possible to determine what the "most similar" existing word would be, and mimic its own rules for making plurals?
For example, one wished to determine the plural form of koopa, a species from the Super Mario Bros universe, be it either koopas or koopa, would there be a way to tell by looking at similar real English words, such as "sheep" or "mouse" (which are both names of species of creatures, like the fictional word, and have special plurals)? Or would the only way to know be to ask Shigeru Miyamoto or another source of Nintendo of America? (Please note that this is only an example, and does not itself need answering)