It's basically a wannabe Pokémon thing. I'm not really sure what the purpose of these miserable little monsters are or who'd want to catch them, but I do know that it shouldn't be too hard if you ever felt that you'd need one of these sickly things in your balls.
I did have a few more "Monsters" drawn out, there was even one named "Kum", but I figured I'd keep him off since he was a Muk who was jacking off. Maybe I'll make another one of these with the missing monsters and mark it as gross. IDK.