The Brown Ferret is one of the two characters in FilmCow's video "Ferrets". He's implied to be a psychopath.
The grey ferret (Harold) enter the scene telling to the brown ferret he's sad for some reason. The brown ferret start singing about the things he likes more in the world to cheer his friend up, but as the song goes on he reveals to enjoy stuff like "Touching Harold innapropriately while he sleeps" and "That everlasting thrill during the final kill, Body dumped in a landfill, got off scott free", or that he believes the Holocaust is a lie; by the end of the song he even insults Harold and threatens to eat his brain one day.
It's unknown whether the Brown Ferret really committed the crimes he mentioned or if he's just making things up, so his status as an outright villain can be questioned.