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Anyway, the plural for Pokemon is Pokemon and that's true of species names also.
Eg. A swarm of Beedril, a flock of Pidgey. Putting an 's' at the end would be wrong. Poke Balls are an exception to the rule.
As pointed out by Mizan, the word Pokemon does not add an 's' when used in plural. Same is the case with Pokemon species. For species, collective nouns are used in place of a suffix.