Moemon refers to humans either dressed up as, or merged with Pokémon. This can be in the form of art drawn by fans or cosplay.
Blaziken with boobs?!
Moemon generally refers to drawn art of girls (though occasionally guys) dressed as Pokémon, usually with the particular Pokémon they are dressed as with them in the image. In lots of cases though they are actually merged with the Pokémon to make a human-Pokémon hybrid (often called pokemorphs). Moemon can also refer to real people cosplaying, but should not be confused with say, someone wearing a full Pikachu suit that doesn't really show any human features.
Pokémon which are already somewhat humanoid are more popular than those which aren't, for example Lopunny and Blaziken are already quite human-shaped and are therefore easier to be made into moemon.