@sumwun, “pokemon” is a plural noun since it names more than one person, place, thing, or idea. It’s grammatically correct to keep it the same, since the plural and singular are the same. Like fish, or deer.
“For the names of certain types of animals, the singular is used to denote both one and more than one individual. Some of these nouns have the same form for singular and plural. The regularly formed plural is used to indicate more than one species, strain, or variety”
https://www.adfg.alaska.gov/static/home/library/pdfs/writersguide_section7.pdf
There’s multiple different Pokémon. It has different meanings. So, “Pokémon” is the plural.