Blend of Pokémon + maniac. Derived from the class of characters in-game called POKéMANIACs, nowadays seen donning POKéMON cosplay pieces (though originally wearing lab coats and portrayed as being somewhat lunatic). "POKéMANIAC" was the term used in the English translation for the aforementioned, originally called 「かいじゅうマニア」 [kaijuumania] ("monster maniac") in Japanese.
Pokémaniac (plural Pokémaniacs)
For usage examples of this term, see Citations:Pokémaniac.